(french listener here, sorry for the bad english).
I keep thinking about how multi-support is a big deal on the MMO Market. Most of the best selling and succesfull MMO's are compatible on both Windows and Mac and I think it is a pretty important feature.
I am in the very same case as Nikole, playing on a Mac because I simply do not have enough money right now to build a decent PC and as an avid MMO player, my choices are pretty limited.
Sure, there aren't so many "Mac gamers" out there but still...(and then again, steam has done A LOT in that regard).
The social component of those games is one of the main feature and you'd want to play with as many friends as you can.
The fact that for example WoW or GW2 are available for Mac has 2 main consequences:
-The games are available for me, so I will buy them.
-I can play with my buddies so we will both be playing more of the game and have more fun.
I'm not sure if I make any sense to you guys but to me, the next successful MMO must be available on both PC and Mac.