Lineage II! I've mentioned it a couple times here. Under-rated game IMO. It's an older game, it released in spring of 2004 I believe (I've been playing since the summer of '04), but it still has some of the best graphics around the MMO circuit. It's always getting updated through it's free expansions, which btw, the 7th expansion is going to be released here in North America later in the year.
I haven't played any other MMO's other then FFXI for a couple weeks, so I can't really go anbd compare L2 to any others on the market. What I can say though, is it's a pretty wide open game. The map is getting quite large, with plenty of areas and towns, although some people would say a huge map is a bad thing: big map, not enough people to fill it etc. Me? I've always been the more is better type so it's all good.
It's an open PvP game, so the only safe zones are towns. If a lvl 80 wants to come to your noobie area and kill your lvl 7 character w/o a reason, it's perfectly within the rules of the game, and you can't do a damn thing about it. Don't be fooled though, that sort of thing is quite rare nowadays as the starter towns are mostly vacated of actually people. The games been out for awhile, so most of everyone has moved on. That's not to say that you won't see people of equal levels, because there are people constantly re-rolling, and the new player joining the game.
What about the population of the servers you ask? There is 10 servers (2 are located in europe). 2 or 3 are "light", another couple are "normal", and the remaining servers are "heavy". I think you understand what I'm trying to say here.
Lot's of items, armour and weapons in the game, I just wish there was a bit more customization when you first create a character.
The main focus in L2 is the castle sieges between rival alliances, which occur every 2 weeks. Pretty fun if you ever get the chance to participate in one. Coming in the new expansion are fortresses. They're mini-castles I guess is what you can call them, and they're siegable 24 hrs a day.
I'll mention the biggest problem in L2 though: bots. There are a lot of them, and they especially come out in force during certain events. NCSoft seems like they're trying to rid the servers of the problem, but it just doesn't seem to be enough. You'll notice them quite a lot in the early stages of the game.
Other then that one glaring problem, it's really a great game. I haven't played for a couple months, but I'll probably reactivate my account(s) when the new expansion comes out.
Here's a couple vids if you're still interested:
Random fan made Gameplay movie. Pretty well done.
http://youtube.com/watch?v=LLiTxP-aqY8
And this next one is the new CG movie of the expansion. A new race is actually being introduced with it.
http://youtube.com/watch?v=Wva0-lW-QIA
Someone else mentioned Warhammer. That seems like a popular choice for some of the people I know when they mention other possible MMO's they'd like to play in the future.