Before trying to define complexity, let me define a related term: "Elegance". Elegance is the property of being the simplest system necessary to achieve a certain desired result. Games should strive to be elegant. They cannot avoid complexity and still be good games. Complexity is good.
Necessary Complexity is good, I suppose, Needless Complexity, bad. What's necessary vs needless largely depends on the purpose of the system. I don't think the purposes of TSR & WotC era D&D, for instance, were tremendously different...
Um, there's more to overall system complexity than the task resolution mechanic.
Nod, it was just one example of needless complexity in the classic TSR era vs the comparatively clean/elegant and consistent complexity of the WotC era.
For example:
Questions - how much text does it take to describe the OD&D Fighter class?
Heh. Do you include the sections of the DMG describing the fighter's potential followers & that Keep he gets to build at 9th level? What about all the fighter-only magic items, those are essentially fighter class features?
But, another way of thinking of it: could you, from memory, easily re-create the 1e fighter? It's exp chart, combat matrix, saving throws, attacks/round, etc?
What about the 3e fighter? Exp/level, same as everyone else. d10 HD, 2 skill points. A bonus feat at first, one at every even numbered level. Full BAB, Good FORT, Bad REF/Will. Hardest thing'd be remembering which are his class skills (Climb, Craft, Jump, Ride, Swim?).
How many choices does one have to make in character creation, leveling up, and play? How many decisions do you make or elements do you have to line up and know before you can engage whatever resolution mechanic is used?
Then, same questions, but for 3e, 4e, and 5e.
The numbers can be HUGE. Years ago I did calculate the theoretical number of unique single-class 3.x fighter builds, for instance, and it was just ridiculous...
...yet the class description, itself, is stunningly simple. You don't experience the complexity implied by the whole universe of choices because you automatically narrow them down quite a bit based on concept.
I did take a stab at a more complete answer, it got really long:
Chargen:
1e: Choose chargen method, the most common, Method IV, IIRC, 4d6-and arrange, so make 5 choices of stat placement. Choose Race, Gender (because it matters, mechanically), sub-race, Class, depending on race, choose a multi-class combination. Depending on class Randomly roll your gp and buy a bunch of equipment, any bit of which may be critically important. Depending on class randomly determine spells known and prepare 1 to as many as 4 spells for the day, or pick an all-important 'optional' weapon specialization, etc..
3e: Assuming point buy, make a /lot/ of choices about your stats. Choose Race, Class, distribute anything from 6 to 44(?) skill points, equipment.
4e: Assuming elite array, make 5 choices about stat placement. Choose Race, class, & PH 'build' - everything else is chosen for you, right down to gear. ;P (No, really, you could cut to the chase like that in the PH.)
Essentials: Choose Race, Class, Background, Theme, Alternate Class features or Sub-Class, Powers, a feat, and skills.
5e: 5 choices about stat placement. Choose Race, sub-Race, Class, Background, skills from background, & skills from class, and spells known and/or prepared. Depending on class, you might also choose sub-class (like wizard tradition) at 1st. You can let background & class choose most of your equipment for you.
So, yeah, in all eds there's a lot of complexity in chargen. In 1e, there was less choice and more randomness.
Level up? In 1e, add up your exp, at the end of each session, possibly add a 10% bonus, divide it by your number of classes, check your next level goal for each of those classes to see if you leveled up, then go looking for training and flush a bunch of gold to actually level up, then look to see what your class gives you at the new level, because every class & level is different. Depending on class, gain new spell levels, special abilities, etc, possibly gain followers, maybe even fight a duel to keep the new level.
3e: No bonus, no dividing among classes though you might have an MCing penalty (rarely), just check one level chart. Pick a class (or PrC! or next level of a class you already have) to take for your next level, determine if you can actually take it without penalty, and look up what that class/level gets you because they're all different. Distribute your new skill ranks to skills, minding the cost/limits of in-class vs cross-class. Pick a feat every 3rd level, or if a class-level gives you one.
4e: Add up your exp, check it against one chart, unless you miss sessions you all level up at the same time, and pick a power from your class or feat and/or stat bump based only on your level, because there's just one progression for everyone.
5e: Add up your exp, check it against one chart, unless you miss sessions you all level up at the same time, check to see what the next level of your class gives you, because they're all different, pick a feat or spells or other feature if you get one; optionally pick a new class or next level of an existing class if you're using the optional multi-class rules in the PH.
While the WotC era has brought in a lot more customizability the net price in complexity hasn't been that high, because needless complexity, like different exp charts and different THAC0 progressions and the like have been done away with.
In play... OMFingG…
1e: Depending on class, memorize some spells from either a short list of known spells or your full list of spells for your level, or just wait, bored, while everyone else does the same, maybe break camp while they're studying books and praying for 10 min/spell level/spell. Tick off rations, light sources, &c used. Declare weapon-in-hand and marching order - if you want any chance of sneaking up on anything, get your halfling or thief well in advance of the party. Wander around in the wilderness or dungeon, carefully asking the DM for every conceivable detail of the environment that might drop a hint about treasure or danger. Either roll % dice (Thief) or d6 (surprise) to determine if you sneak up on anything (or vice versa). Each side (or player, depending on your DM, the rules aren't crystal clear and some make more sense one way, some the other) rolls a d6 to determine surprise, what determines surprise depends on the character and the enemy and can result in either, both, or neither side being surprised, or some of one side or the other being surprised or not. Each segment you're surprised, the enemy beats on you like it was a whole round, 'cept for spell casting. Probably. Again, the rules were neither consistent nor consistently applied by DMs. Once you're in combat, either attack, or move and attack, or charge, or set a weapon to receive an opponents charge, or cast a spell, or draw and maybe use a different weapon, and roll initiative, again, maybe on a d6 for the whole party, or maybe a d10 for each (it was a super-common variant), and apply casting times and weapon speed and factor in attacks per round to try to figure out what order things happen in, and, maybe even, in some cases, get a bonus attack for tying but not winning initiative... if you attacked, roll a d20 to hit, apply various modifiers based on the weapon you're using, the circumstance, the armor they're wearing, and tell the DM, who will add any secret modifiers and consult a matrix to see if you hit, if you did and the monster could be harmed by your weapon, roll damage based on the weapon and the size of the enemy you hit, and add more modifiers... if you're casting a spell, and you didn't happen to get hit in the middle of casting, look up what the obtuse Gygaxian prose of your spell says, while the DM looks up the extra notes about how it works in the DMG and everyone else settles in for a long argument about what happens... If your next challenge isn't a combat, describe everything you do in excruciating detail, including every argument you can think of about why it should work (and why your character should know it should work), unless you happen to have a 'special' ability that lets you just solve it with a pass/fail % check, or a spell or magic item that you can make an argument (usually involving middle-school/PBS-special/comic-book science) will obviate the challenge in some way.
...oh, and I almost forgot, do the same for your henchmen and hirelings! (but be careful with them, because the DM is constantly adjusting their Loyalty and Morale...)
3e: Depending on class,
memorize prepare some spells from either a list of known spells (wizard) or your full list of spells for your level (Cleric/Druid), or just wait, bored, while everyone else does the same. Tick off rations, light sources, &c used, unless you've found/made/bought some infinite-resource magic item. If you want any chance of sneaking up on anything, get your highest-stealth character well in advance of the party and have him make a lot of stealth checks. Wander around in the wilderness or dungeon, declaring every skill check that you think might reveal a hint about treasure or danger. If any challenges come up, everyone roll every relevant skill until someone succeeds (or just take 20). Or just cast a spell, probably from a scroll, that obviates the combat. If the challenge was an unexpected combat, roll contested stealth & perception to determine surprise. Then roll initiative. The highest initiative caster picks the right spell from the list of every spell he knows (spontaneous) or every spell he has prepared (which might be dozens at higher level) /and hasn't cast yet/ or magic item (from everything you've collected or made) for the fight to bring it to a swift conclusion. If you know about a combat in advance, layer pre-cast spells until the challenge is obviated before initiative is even rolled. If anyone gets hurt, pull out the Wand of Cure Light Wounds and heal them back 1d8+1 hps & six second at a time.
4e: Decide on something you'd like to do, or just board the railroad of a linear published adventure. If there are difficulties on the way, the DM may make it a skill challenge that you each take a turn and try to apply something you're good at too until you succeed or fail ("forward"), if it involves a fight, and you want to sneak up on 'em make a group stealth check (DC of their passive perception), if half of you succeed, you gain surprise (one standard action each), then you roll initiative, and, in initiative order, decide where to move, maybe take a minor action, and (mainly) which of your several attack powers to use - if you attack or cast an attack spell, roll to hit (with your predetermined attack bonus) vs the targets defense, and roll the power's damage dice, maybe with resistance deducted. After, everyone takes 5 (minutes) and spends healing surges @ 1/4 max hps each, to get back to full.
5e: Depending on class,
prepare some spells from either a list of known spells (wizard) or your full list of spells for your level (Cleric/Druid), or just wait, bored, while everyone else does the same. Wander around in the wilderness or dungeon, declaring every action you think of that might reveal (possibly with a successful check) a hint about treasure or danger. If the DM does call for a check from a character and he fails (with no particular consequences), everyone else piles on and tries the same thing until they succeed, if it involves a fight, and you want to sneak up on 'em make a group stealth check, if half of you succeed, you gain surprise, then you roll initiative, and, in initiative order, decide whether (there's often not much point) & where to move, and declare an action, which may or may not also entitle you to a bonus action, and, if you attack, roll a d20 + bonuses vs AC, or if you cast a cantrip or spell either do the same or call for a save from the target(s) which is also d20 + bonuses but vs your save DC, or do whatever else the spell says. If you hit, roll weapon damage, or if your targets fail their saves, do whatever the spell says. After, if you have an hour to rest, you can spend (roll) some HD to get some/all of your hps back.
