I figure it's good to have a kinder, gentler version of the 'bad MMO things', because it's easy to forget how many MMOs have some serious gems. And who knows, if people chip in maybe some of us will mention bits in games that'll get others to try them.
Here's what comes to mind for me.
Dark Age of Camelot
The first MMORPG I played heavily (UO and EQ just didn't bite me), and one that still holds a fond place in my heart.
1) Effective and suprisingly balanced RvR. It wasn't perfect, but Mythic got far more right than they got wrong (and amazingly so few tried to stand on their shoulders and instead implemented bland tack-on FFA type PvP)
2) Extremely powerful skill trees, ones that force serious choices and really define your character - These aren't WoW's skill trees.
3) Very flavorful classes - Despite being an old old game, they've still arguably done the Sorcerer, Minstrel and Palladin better than any game since and there were some gems like the Theurgist, Animist, and Skald that were a blast to play.
4) PvE Crowd Control where skill meant something - if you were very good then the group you were with would have to find much easier prey when you left.
World of Warcraft
C'mon, we've all played it
1) Refinement - Blizzard really does work on the quality of their product
2) Solo vs Group balance - You constantly feel like you CAN solo, but can just do a bit better with a group.
City of Heroes/Villains
One of those games I didn't expect to like nearly as much as I did, I still go back to it occasionally
1) SPEED! - Other games pale compared to it. Travel can be genuinely fun (with super jump or super speed) or fast (teleport and flight).
2) Genuine 3-D - Is there anywhere you can't go in that game? Who hasn't teleported/flown/gone to some out of the way place way up on the top of a building because you CAN? it's a far cry from games with hills that are just a little to steep to climb (err, pretty much all of 'em)
3) Amazing Customization - I remember making
Captain Clash as a joke during beta and suddenly having him be surprisingly wanted in groups, and so many times I had a concept and against all odds I could make it (Commodore 64 the zombie-pirate-robot with a horde of ninjas? check! The Visible man, who only has invisible skin? Check! The Meloncholy Emo Boy? Check!)
4) Very dynamic action - depending on the group you'd often develop new tactics on the fly
Everquest 2
Ironically, EQ2 still feels the same to me as it did when it came out - a more wide open and less tightly refined WoW
1) Heroic Opportunities - a very good group-friendly addition, somewhat re-implemented in LOTRO, but not nearly as well IMHO
2) The Isle - having the very flavorful starter area that's separate from the rest of the world was nice, having it be where the trial accounts went was nicer, making it so that trials could only broadcast to the isle (and not spam gold messages to the rest of the world) was priceless.
Eve Online
There's nothing like it in the MMO world, is there?
1) Scale - The first time I flew by a carrier in my cruiser . .wow.
2) Real time training - I love this, even though I know it drives some people nuts. The general idea that you can be training even when you're offline is great fun, and makes the entire game more cerebral and less obsessed-grindy-teen frinedly.
3) Ownership of parts of the universe - while sometimes the PvP got ganky the fact that corps could literally 'stake out claims' and build huge bases in starsystems (that required a whole economy to produce) was amazing.
A Tale in the Desert
One of the best games nobody's played, possibly because you don't get to hit people
1) makin' things in the real world - the first time I made a sculpture and realized that I'd just made part of the world that everyone else could see was pretty nifty. Yeah, it was most of the game, but it was very cool
2) Voting - in the game the player base would literally (in game) vote on changes to the game.
Tabula Rasa
My current biggest timesink
1) fun combat - I forgot what it was like to have fun in combat over the years

You know those times you'd run by a few green Murlocs in WoW and kill them just out of spite? Imagine being able to do that with a shotgun, a robot by your side, and turrets. . or maybe a flamethrower. .giggling all the while.
2) fast travel - you don't spend much time getting from place to place, you spend it actually being there and killing or getting killed
3) The bad guys took the base? - the first time I finished a quest and discovered that the base I had to go to turn in at was taken over was priceless, as was the convo with Ms. Me about it. I didn't see that coming, and CP battles can be fun
Sword of the New World
Yes, it's Korean style, which means grinding and obsessed people and stuff, but it had a couple of gems
1) being your own party - in SotNW you play a party of up to three people, so you can be a balanced party or be three elementalists for AOE goodness
2) quirky fun music - it's the Katamari Damacy of MMORPGs
Dungeon Runners
NCSoft does free!
1) Fun Item Names - Baby Seal Gloves of the Newt? Health Potion of the Daring Noobsaur? Chilled Rainbow Ghost Threads of the Wallaby? Algor's Frozen Turkey Leg Club? Heck, my main has a giant Pizza Cutter, which I'm waiting to bring into that level full of lawyers
That's a good start I think. . what gems has everyone else found?