Limited Skill Deck-Building
Most MMOs have traditionally been of the model that every skill your character learns is available to you at will. Certainly, some skills require a particular item to work. And some MMOs don’t really give you that many in the first place. In fact, most MMOs have a very hard time giving you convenient access to more than a dozen skills, because once you have one bound to every number, the rest generally have to be accessed via binds, macros, or clicking an icon with your mouse. WoW and now SW:TOR are the champions of this last fact: by max level you can easily have over 40 power icons on your screen, each of which is useful frequently enough to leave it there.
But, given the way your hands work, the only ones convenient to use are the ones within easy reach of your fingers. In games with dozens of useable powers, high-level encounters might be balanced to need a lot of them. So your ability to play the high level content really comes down to having an amazing spatial memory for your skill bar, good recall about what each of your abilities does, and tricks for getting there fast. I’m sure there are people that love this type of player competence being required to play well, but it’s not really that friendly to a mass audience. I suspect the vast majority of players almost never use more skills than fit in their first row of quickslots.
The first place I saw this directly supported was Guild Wars 1: you can only have eight skills active on your character at a time (accessed with number keys 1-8). You might know literally hundreds of skills, but you have to pick eight to take into an adventure (you can freely arrange them in town). Theoretically, not many skills are drastically better than other skills, but each varies in cost, time to use, cooldown, and effects. Some skills are better if you can use other skills to cause an effect (e.g., a skill that does extra damage to burning characters, but doesn’t, itself, set them on fire). Others are more valuable based on party composition (e.g., a skill that gives bonus HP to every party member for each enchantment is more useful in a party that uses a lot of enchantment buffs). Still others are more useful if you know what you’re fighting (e.g., armor against earth elemental damage is incredibly useful against specific enemies but completely useless against most others). So choosing your skills is a lot like building a deck in a collectible card game: you’re looking for synergy and strategy over raw power.
It’s not surprising that Guild Wars 2 is keeping a similar system. But it’s not the only MMO of this generation that’s doing so: both TERA and The Secret World use a similar mechanism as well. GW2 has changed it up from the previous game: you still only have a limited number of skills, but half of them are based on equipped weapon, one is always a heal, and one is always a high-level elite skill (reducing the deck building complexity considerably). You also often have different class-based effects on F1-F4. This theoretically gives the player less freedom, but probably makes balancing content significantly easier for the designers (currently in GW1 there are skill decks with synergy so good they can allow a prepared player to safely solo elite group content). Meanwhile, TSW is much closer to GW1, in that you have complete freedom to choose your skill deck. Their variation is that skills are unlocked via a tree structure (rather than just captured or purchased individually) and you have an additional bar of skills that aren’t activated, but give you passive bonuses. But all evidence points to skill synergy being at least as important as in GW1. Finally, I only played TERA briefly, but it seemed to be using a similar system more for ease of access than anything else: I recall that you could only put abilities on the first six number keys and the first four F keys, ensuring that you’d never have to reach across the keyboard to use a power.
In general, I really like the trend that seems to be happening. Having lots of skills available at once really makes it necessary to pay more attention to the UI than to the 3D world, even if you can manage all those abilities. It also seems to make a “gotcha” encounter design more common, where you’ll normally be fine with just your first quickbar of skills, but you’ll periodically hit a wall of difficulty if you don’t remember some skill buried on bar six (“Oh, right, I can stun droids! That encounter could have been a lot easier!”). Knowing that you can only have a small number of your total skills available at one time makes it much easier to just pay attention to the 3D game and feel safe that, if a skill selection is doing well in an area, there will probably not be something you forgot to use in a later encounter.
The one downside to it is that it has no particular simulation rationale: it’s entirely a game mechanic, and there’s no explanation for why your character can’t use any learned skill in a pinch. But, given that most MMOs have hugely more severe violations of such immersion, that’s probably not worth getting annoyed about.