Something I'd like to add to the "codified vs freeform abilities for monsters" discussion is what I like to call the "Giant Octopus Riddle".
The giant octopus monster is an iconic fantasy encounter. It goes something like this:
This is the artwork for the 4E Silt Horror, a Darksun variant of the big bad that I consider particularly successfull.
When you think "giant octopus encounter", a few things come to mind:
- it moves underwater, unseen most of the time
- its tentacles attack from every direction
- it grabs people, pull them into the air, and drag them underwater, to the creature's mouth, or drop them from high altitude
- it swallows people whole
- it has ways to evade location
etc.
The 4E SH manages to do all that by bending (never breaking) the rules of the system a bit, and being implemented as an Elite monster (for the core) and a cadre of Minions (the tentacles). All tentacles have different abilities, that let them do all that stuff from above.
Now, let's assume we want to do the same in a game where those abilities mustn't be unique to this specific monster, and you have to go by with the same abilities players use.
Now, the first thing we see is that the Octopus has to be able to grab people. How does he do it? Well, it's really big, and strong, and that must play out in Grapple checks, right? So, we follow the general guidelines for grappling and we either find out:
A) being big and strong is very good for grappling, and grappling is a viable, useful strategy.
B) being big and strong doesn't matter much for grappling
C) being big and strong is very good for grappling, but grappling is generally a waste of an action, expecially for a very big monster
A) is 3E. What happens here is that since grappling isn't a "contextual" ability, and the octopus isn't any better at it or doesn't get anything out of it, then every big monster will go for grappling. Same-ness here happens because the option is there, available to all, and if you can pull it off you should. So all big monsters grapple, and the uniqueness of the giant octopus is gone.
Now, 5E is a bit of B and C. Big creatures aren't as good as they were in 3E at grappling (at least, players have a more solid escape chance than before, as you basically never get truly impossible DCs) but they also get very little out of it. Sure, the big octopus can grab people and maybe pull them up - but he's simply immobilizing them. And if I want him to do stuff with the grappled creatures, it's pretty hard. I have to reference the books over and over, and things I could do with the 4E, exception based version can't be done here. For example the 5E octopus can drag the grabbed player, but only at half speed, during its action, and in one direction. Since in the 4E version I have tentacles as individual creatures, I can have them drag people in different directions, at different times and doing different things when they do. The octopus-specific bits add both complexity
and freedom to the encounter.
Let's look at the zombies from above. Sure we picture zombies as grabbing people, and then biting and devouring them. So, let's play with that a bit, ok?
Now, the zombie does his grapple check and... the target is grabbed. So, what happens now?
Really nothing. Nothing that wouldn't happen if a goblin had grappled him. Or an orc.
If "now the target is immobilized" is an interesting effect, then those creatures should aim at pull this off the same way. Anything "interesting" that comes out of the "the zombie has grabbed you and is dragging you in the middle of the horde" would apply to "the goblin grabbed you and is dragging you in the middle of the orcs".
So, we're still gonna pull that off because it's tematic and good role, but we may be making the encounter less
scary in the process.
Let's look at how the exception based handled it.
The first iteration of the zombie is equally terrible. He gets a grab action, but that's that. And he wastes an attack using it, to do pretty much nothing. He suffers the same problem.
So, the designers wise up and introduce variants like the Grasping Zombie. Here we start reasoning - he has an attack that does damage AND grabs you, and if you're grabbed, his basic attack does A LOT more damage. These zombies will aim at grabbing you, and for a good reason. The Tainted Zombie is another interpretation: here you have one round to escape the grab, or he'll bite for massive damage.
But let's move further down the line. The Silt Zombie is even better: here you have attack, grab, and mechanics for what happens while you're grabbed and he's trying to chew your face off (ie, each turn you fail to escape you take some damage) built into a convenient, easy to use package. No referencing multiple books and different rules, all numbers and DCs in one place, everything happens in a single, specific attack.
This is, in my opinion, the best of both worlds: you're not overcomplicating the base rules to make sure DMs can leverage them in specific cases, you're not asking DMs to go back and forth among books, you're getting precisely what the encounter evokes with all the rules and numbers you need in a few lines of text. 5E definitely needs a lot more of this, and I hope future MMs will show it.
As someone who works on designing roleplaying games, I often joke (but it's half a joke) saying that my ideal system should have 3 pages devoted to actual rules, 30 pages devoted to player options, and 300 pages devoted to monsters. Put all the rules inside the monsters, and the player abilities, and keep the basic system lean, flexible and easy to understand. This way all the complications are invoked only when a player or a monster ask for it, and thus only when absolutely needed.