I've been playing Dota 2 / Hereoes of Newerth on my 13in mac air for years just fine. My only gripe is the mouse acceleration on OS X... the mouse feels horrible and you have to get third party software to make it only slightly better.
I don't know what it is. The Macbook's track pad is miles above its peers -- an absolute joy to use. However, the second you plug a mouse in, the cursor becomes an unusable, inaccurate, enraging piece of garbage.