Not all MMO are equally polished, fun or accessible. However even in the most criticized games there are great game play.
Among others here's a list of good mechanisms :
RIFT: class system. Being able to choose 3 talents trees and associated skills among 8 for each of the 4 classes. [Cons: won't likely ever be balanced by the devs... but by the players]
WoW: dual spec talent tree option [Cons: can create issues for sharing the loot. Too expansive for more casual gamers?]
SWG (pre-CU): lack of levels. Anybody is welcome to join any group. [Cons: can be confusing for the WoW generation of players]
SWG (CU, pre-CU): Skill trees instead of classes, and ability to respec into any template. [Cons: apart it was never balanced, can be felt a grind to go back once again through a former talent tree. ]
LotRo: fellowship maneuvers, making combat more more coordinated. [Cons: can be felt as complex to learn]
AoC: adding combos in combat. [Cons: not as convenient as a console combo system. Can be felt as password protected skills]
AoC: fatalities, the joy of beheading your enemy... [Cons: apart ESRB rating, bit too much random]
DDO: Character's collision and positional combat [Cons: you need to be coordinated to block a passageway. Creates lag in non instanced zones cf WAR.]
SWG: crafting from resources gathering to experimenting. [Cons: painful to manage the resource inventory. Click fest to grind. Not so easy at higher level]
TOR: crafting mini missions [Cons: none so far but that we don't know what's the real game play behind]
WoW taxy system, convenient to move around and get a drink while traveling [Cons: multiples hops can last way too long]
AoC and SWG: Character creation tool. [Cons: too often gear don't fit extreme body presets.]
SWG: in game access to the character creation tool (Image designer) to modify the character's body. [Cons: can be immersion breaking for some]
AoC: UI art, very clean and giving a distinctive look to the game [Cons: the UI features are one of the worst AAA MMO wide]
WoW: UI addons [Cons: in the end some mods become mandatory]
WAR: default UI features: show/hide/resize any of the UI element [Not as customizable as WoW with addons]
LotRo: Social clothing tab, don't need anymore to look like a twin or a mismatched walking clown [Cons:(for WoW devs) you are not obliged to display your high end gear at the AH]
GW2: 3 zones gear dyeing areas. [Cons: can end with weird choices, comes with RMT]
Aion: transferring stats from an item into another. [Cons: The noob pants you are equipped with might not be that bad...]
TOR: fully voiced over dialogs [Cons: not totally skippable. Can be an issue for players not much into story]
SWG: Housing furnishing [Cons: Loads the game DB with thousands of items and heavy queries]
I'm certainly missing quite a few so what's yours?
Comments
CoX's Sidekicking. A very social feature which lets players play together regardless of level difference (yet retaining the tight difficulty-balancing that's crucial to a game's fun.)
WOW's Dungeon Finder. Finally grouping became painless. Who wants to waste time on non-gameplay?
"What is truly revealing is his implication that believing something to be true is the same as it being true. [continue]" -John Oliver
FFXIV: Fishing (especially being able to do that on hdtv with a wireless remote lying on a bed).
"Only in quiet waters do things mirror themselves undistorted.
Only in a quiet mind is adequate perception of the world."
Hans Margolius
Not sure if I can say it's my favorite, but I really like EQ2's encounter system. It's a shame they didn't take it further than they did, but I think it's something that could be expanded on to make PvE AI a lot more interesting and challenging.
If you mean a feature from new MMOs, then that feature is....
/quit
Corpse Runs and Hell levels.
No Signature
Favorite mmo feature? RIFTS!
Darkfall - All moves are fully aimed, Full loot, Player owned cities
Open world PvP
I liked Mabinogi and Eve's classless concepts. Though I don't like skills for a set time in Eve, I do like being able to learn anything you want that is available.
Edit: Honnest reply
Community and grouping
Im done with wow, but the seemless world was one of my favorite things, really made it seem like a world.
So many mmo's dont implement this, admittedly it is probably decided very early on in the development cycle as its all about the engine being used, but there is nothing like zoning from one zone to another to ruin immersion, especially when there is not even a world map, only a map for each zone.
open world pvp or fatalities