Guild Wars.
It doesn't bring the Sandbox-Player Owned Content, but does have the multi class feeling you've suggested. What I like about Guild Wars (besides being able to changes classes on the fly) is that it isn't gear or level based. You progress by unlocking skills (spells) and grinding titles. Playing a 55hp Monk was the most rewarding class I have ever played.
Look out for Guild Wars 2. It might be the best MMO I've seen in awhile.
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