As for my checklist:
BECMI D&D:
- Tiers
AD&D 1st Edition:
Nothing except the 'feel' of the game. To whit:
1. This is a game for adults. It won't go out of its way to offend, but it also makes no apologies for what it is. If that offends, then that's too bad.
2. The game takes itself seriously, but not too seriously.
3. The old-school vibe. More Elric/Conan/Lankhmar, less Drizzt/Wheel of Time/World of Warcraft. (I have nothing against those latter sources, but see 1)
AD&D 2nd Edition:
Again nothing, except that this shows the sort of density of options I'm looking for. (That is, core 2nd Edition.) Not too many, not too few, but a nice medium.
3e:
- Basically, the core of the system: roll d20 + mods to beat DC for most rolls; roll d% (with no modifiers) for fixed probabilities.
- The above also covers a multitude of little details - skills have ranks, characters have feats, the alignment system is intact, monsters work much like PCs, and so on.
But definitely not:
- Too many 'bad' options. Every option should be at least reasonable; Bards and Monks were too often far too weak, likewise Half-orcs and Half-elves, and there were too many armours that no-one ever used.
- System mastery. A small amount is inevitable, but too much means that experienced players can't sit at the same table as newbies without one group or the other being dissatisfied.
4e:
- Per-encounter balancing.
- Class roles (although I would neither be so rigid nor so explicit about them)
- The encounter design philosophy, and especially the Minion/"Normal"/Elite/Solo split and monster roles.
- The Skill Challenge concept.
- All characters have some ability to heal themselves. I wouldn't use healing surges as such, though.
- Less fragile 1st level characters.
- Rituals.
- Passive Perception (and Sense Motive) defense.
Star Wars Saga Edition:
- Talent Trees. These strike me as a very elegant way to allow players to customise their characters, and also a neat solution to the "dead levels" problem.
- The condition track, but not the damage threshold.
- A damage bonus for high BAB in place of iterative attacks. (Actually, this would probably be handled by adding damage dice, rather than adding a damage modifier.)
Trailblazer:
- Combat reactions. Another elegant way to consolidate Immediate actions, the ability of some Elite/Solo monsters to act when it's not their turn, and Attacks of Opportunity.
Pathfinder:
- Combat Maneuver Bonus and Defense.
- The skill system. Also most of the skills, although I probably wouldn't bother with Fly, and would certainly add an Athletics skill to combine Climb, Swim, Jump and Run (not currently a skill).
Arcana Evolved:
- Probably the division of spells into Simple and Complex spells.
WFRP 3e:
- More abstract combat - no need for minis/less micromanagement of positions.
Other:
- "Stunt Elements", allowing the player to declare a single complex stunt and resolve it in a single roll.
- "After the fact stunting". A character who misses his roll by a couple of points can "rephrase his action" to tie it to one of his stunt elements to gain a small, but crucial, bonus. Repeated use of the same element gives a decreasing bonus, of course.
- Mana points, consolidating all the "impossible things" that a PC might do into a single pool.
- Split the hit point and mana point pools into two. Hit points have a 'quick' pool (that refreshes with a short rest) and a 'dead' pool (which only powerful healing magic can refresh during an adventure). Mana has a 'slow' pool (that refreshes with a short rest) and a 'live' pool (that drops to 0 with a short rest) - characters can take an action to increase their 'live' pool during an encounter.
- Shields don't give an AC bonus - they give a pool of bonus hit points.
- More abstract (and simpler) encumberance system.
- Reduced emphasis on ability scores - modifiers are +1 per 3 points, not 2; ability damage/increases apply a condition, but do not directly change the modifier (as with hit points); remove the +1 to a score every N levels; remove most spells and effects that modify scores; scores are generated using 4d6-drop-lowest in order by default.
- Reduction of choices at 1st level to speed character generation, especially for new players. The first feat is gained at 2nd level; Spellcasters get a fixed set of basic spells at first level; ability scores generated as above.
- Characters attain their maximum BAB, skill ranks, and capstone powers at 15th level. If they continue beyond that point, the game is more focused on legacy and empire building than on accumulating ever-greater powers.