I left defaults on except no shadows or AA. On Act II normal without issue so far. I only play single player and I don't play often so I cant say how it'll be later in the game yet.
As bogatyr said disabling shadows / AA is really all that's needed.
I'm farming Act 1 inferno and have had zero issues at all in regards to playing. I even have a few settings set to high.
Only thing that ever concerns me is playing at a high temperature. I do have a laptop cooler though with 3 fans to try to help out and it at least keeps the body quite cool.
I'm at 20-30 FPS as well.
EDIT: The MBA I have is the 2011 11" / 1.6 i5 / 128 SSD / 4GB ram.