Well, the suggestions above are great. I myself keep intending to return to EVE, which is probably the greatest of them all if you want the ultimate and are willing to pay for it.
An often overlooked game you should consider is Perfect World, pwi.perfectworld.com which resembles AION, though I think the truth is AION resembles it. PW has been out in the US and Europe for a bit over a year.
Very deep game, good community, graphics arguably the best in free mmo class. Flying by level 30, or right away for the Elves. Swimming, underwater combat, aerial combat, extensive beastiary, crafting, land mounts at 40, over 5000 quests, 3 races each with its own starting area, expansion out this Winter with a 4th race and two more classes for a total of 8. Mail system, auction house, bank; all you would expect. A bit more "geek" than WoW, stupid people need not apply.
Free to download, free to play; and honest to goodness you really do not need to spend real money in their Cash Shop. No bots either, unlike the unfortunate AION which is being botted silly. Here is how it works. There is an ingame money called Coins. Coins in large quantities are required to buy ingame items and keep rolling. The Item Mall only takes gold which can only be purchased from the Cash Shop for real money. There is no way to transfer gold directly from one player to another. But, the Auction House has a gold exchange where people who have bought gold can sell it for ingame coins. So if you're a good player and about half smart, you can eventually afford to buy all the gold you want for ingame coin and get stuff from the Item Mall if you so desire. So, it really is free to play.
http://www.youtube.com/watch?v=eKewL8b0B5E&feature=related This video just shows an elf priestess flying through the incredible scenery, eventually to turn in a quest. The graphics quality is lower than WoW or GW; but what they do with the graphics in PW in terms of scenery and eye candy is simply fantastic.
I've gotten a Venomancer and a Barbarian up to mid thirties. The game is very very good.