I've played many free to play MMOs and at present I don't believe any of them can offer the level of quality and detail you may have become accustomed to. However, there's some really nice ones out there, and depending on your taste you probably can find a game you'll enjoy:
Rappelz, Fly For Free, Corum Online -
www.gpotato.com
Fiesta Online, Secrets of the Solstice -
www.outspark.com
Dungeons and Dragons Online -
http://www.ddo.com
Those are just examples. The acronym MMO is being attached to pretty much any game that offers online play at the moment. It's an incredibly misappropriated term so prepare to go looking through loads of crap before finding a proverbial 'diamond in the rough'. Lots of Korean games, so if that's not your thing you're already limited in your choice. I'd say check out the link below and invest a bit of time, because an alternative to WoW is hard to find. It's definitely the leader in its genre.
List of MMOs:
http://www.mmorpg.com/gamelist.cfm/gameId/0
I personally love DDO, it's a great getaway and the quality of the game is very good. You can tell that the developers were committed and driven. What's especially good about it is that (as OP stated) a lot of people taking a break from WoW or other games see it as a haven where they can go to get away from the guild / community / OMGNERF messages / etc. DDO is a great alternative although I would recommend EvE Online if you like killing so-called b*tches :P
My personal preference:
http://play.eveonline.com/en/home.aspx
Hope this helped, enjoy!