Final Fantasy XI is another you should consider. Because of the fact that you can change jobs on the fly, there are always higher experienced people leveling low level jobs in low level areas. It's very simple to start up and playing, the there is an extreme wealth of end game content.
FFXI is based on partying, there is almost NO solo play. If you enjoy playing with others, this is your game. Huge HNM kills (Huge Notorious Monsters), high level god kills and high level areas are abundant, and there is fun to be had at every level.
FFXI second largest MMO in the United States (Folllowing WoW), although it doesn't compare to the Korean based MMOs (Linage 2 has almost 4 times the members of WoW and FFXI combined. And the dual nature of the game, with large player bases in both Japan and America lead to party options for all players at all times.
The environments are beautiful
Peaceful
Awe inspiring
And sometimes down right creepy
Im a 66 RNG/NIN on Ragnarok, if you would like to play, I can set you up on my server with an established lot of good players.
"If you watch television news, you will know less about the world than if you just drink gin straight out of the bottle."
--Garrison Keillor