DOTA2 is currently absolutely not pay to win---the trick is getting an invite code.
Every other free MMO that I've seriously enjoyed actually has a pay to kick sand in your enemy's face, but not a guaranteed pay-to-win. World of Tanks is one in which you can pay hundreds and still be out-skilled in a public match, except at the most competitive stages in which people are waging money for a chance to win in-game money.
Most other MMO's are pay for additional content. LoTR Online and Puzzle Pirates are probably such, am-i-right?
I wouldn't call that pay-2-win exactly, although the in-game rewards from working the "additional content" probably give an advantage. Is that really pay-2-win?
Exception: the Civ game on Facebook is definitely pay-2-win.
The worst case scenario is when players are engaged in a perpetual spending conflict; keep spending for a temporary gain over each other kind of like a "keep up with the Joneses". That's not really a game in my opinion.
However you cut it, it's a price per happiness thing. Everyone has their own comfort margin on that. Whether the game is subscription, one-time purchase, or free-2-play with some purchasing of consumables and unlocks, it's all about the same end effect that you spent X dollars for Y hours of fun.