Honestly, and I hate to say it because I'm really not a fan, but I think in terms of MMO influences, WoW has had a grand hand in creating what people expect out of MMO's any more.
To me, this isn't good, and frankly, WoW borrowed MANY of its ideas from predecessors like EverQuest. But WoW incorporated alot of elements - ranging from the UI layout to controls - that worked SO well for itself, that many games since have copped in on that experimentation that WoW used to get their perfect gameplay down.
Personally, the most influential MMO for me (as an indie developer) is definitely EvE Online.
There truly is nothing quite like EvE.
It is as sandbox as an MMO can get while still keep its genre true, and the learning curve is difficult, but once you get time in, the game keeps you coming back. EvE is honestly the ONLY game that I ever constantly go back to, even after not playing for a year or so. It has that much 'umph' in its freestyle play, it's utter open endedness, its grandeur in size and design, and its ability to get the adrenaline going.
Honestly, I don't get pumped with games, but when I am flying solo in EvE and warp onto an unsuspecting person, flick on the autocannons and lock their ship down, the time when your ship is whipping around in an orbit with your enemy in tow, watching yourself and your enemy tear away at eachother, while managing your ship and while making sure they dont have backup inbound, is amazingly exciting. Something hard to pull out of me is a thumping heart in the heat of action, EvE does it. Not to mention EvE truly makes you work for everything you get, you can get into a good corporation who will fund your adventure, but solo, you strive and struggle for everything you get, and that to me makes the game all that much more fun. And even makes losing all that progress fun!
Also, there is NO other game with battles the size of those you will find in EvE. Thousands, and thousands of players get into the same war in the same place, all on your screen (EvE is a single server for all 500k+ players). Such immense battles can even cause time dilation, where realtime and gametime become so split, simply because there is no way to handle that much action, an hour in game can be four hours out of game...its incredible.
The only gripe I have with EvE though is that it IS hard to get into, and you really need a buddy or two to get yourself settled and comfortable in game.