Game mechanics: Armageddon was famous - or infamous - for its insanely powerful, world-changing long-list magick guilds. They split them all up into quadrants, so there no longer exists a full-on sorcerer, or wind mage, etc. Plus they eliminated the option to play shadow, void, and energy mages. They also now allow more people to play the quadrant-based mages, so it's not nearly as "special" as I feel it used to be, since now it seems as though everyone and their brother is either a known or secret mage. There are still dozens upon dozens of totally playable and fun options for play though, that don't revolve around magicks.
Feature set: lots of depth. There is a robust crafting system, the original DIKU-based combat system has been tweaked to better fit the RPI genre so it's still recognizeable to old-school hack-n-slash MUD fans but more RP-friendly. Mounts and wagons, hundreds of different skills - though through class/subclass options, you have to choose which skillsets your character will eventually "branch" and which he'll forgo. The help files on the main website are accessed easily through the search engine, with plenty of cross-referencing. And they are replicated in the game itself using "help "
Game world: There is a main city, a couple of main trade centers, some less-travelled trade centers, dozens of different hunting areas for those who want to hunt. There are lots of hidden spots, secret areas, places that can only be accessed in certain ways. There are also lots of coded and non-coded ways to earn your character's keep: join a clan, forage for stones/salt/food, get hired as an aide by a noble or templar and jump face-first into politics, form your own hunting crew or join someone else's, or join the criminal element and burglarize your way to fame and fortune. Characters can eventually rise in influence/wealth to become "important" whether in coded rank in a clan, or opening their own shop (which takes a LONG time and LOTS of work but it's definitely possible and has been done recently). Or you can just play a character who isn't trying to become important, but instead is just trying to survive and not starve to death or piss anyone off badly enough for them to get murdered. So plenty of options for movement and improvement from your character's first moments in the game til their death.
Community: this is a sore spot, but it's improved a bunch, at present. It has phases where it feels like drudgery to read the game forums. Usually it's just a couple or a few players who are hell-bent on finding fault, and will twist reality to fit their idea of "everything that's wrong with the game." Sometimes there really ARE things wrong with the game, but there are ways to seek resolution that don't involve pissing in everyone else's Cheerios. There are some people here, as in every game, who just want to piss in everyone's Cheerios. They do, they eventually get bored of it or the staff eventually shows them the door, and they go elsewhere to whine. Some of them even get tired of whining and return to the game. It's a very emotionally-driven game, so you have to expect a lot of passion among the players. While it's great that people care so deeply for Arm, it can be a curse at times, when things don't go the way a player was hoping it would go.
Staff: vastly improved from even as recent as a year ago. There has always been the steady thrum of solid, sensible, helpful, courteous staff members, but it has occasionally been interrupted with significantly unpleasant individuals. They weren't bad staffers necessarily. They were just really bad at the staff-player interaction game, and probably should've stayed in the background and let other staffers take over the player relations end of staffing. There've been a few doozies in the history of Armageddon, but you have to expect that with a game that's been running since the early 1990's.
Roleplay: I've played GemStone III, two MOOs, Legends of Future Past, a couple of DIKU RP-encouraged/enforced games, Armageddon, and the first incarnation of Shadows of Isuldur. Of all, I've found Armageddon's roleplay to be vastly superior. The emote system is detailed such that it's easy to really "put" yourself into your character and bring the game world to life. But at the same time, it's absolutely fine to simply nod, if that's all you feel your character would do. You aren't expected to throw out emotes for every little movement. You're just expected to try and make your character believable within the scope of the game's genre: post-apocalyptic desert planet with scary-magicks (as opposed to disney fairy godmother magicks) fantasy. Politics abound, whether you want to be a hero or antagonist, and there's plenty of both to latch on to, or to mentor your character. You never really know who your character's enemies are, sometimes not even at that one moment when you hear the death-BEEP and see the login screen saying "Welcome to Armageddon!" Pro-tip - death by assassination is infinitely more fun than death by mob #47934!