Image quality. If they deliver stunning quality then they are a guaranteed purchase, if not then it's a no go.
So far I've been disappointed with the 24 & 28mm IS, while they have better quality than the lenses they replace by far, the way people say they have L image quality is misleading. They have the most entry level L quality and their L counterparts primes are much better.
The 24-70mm IS F4 looks to be a real winner though, it should have the highest IQ for any normal image stabilized zoom and the macro is a nice bonus.
Image quality is the single most important factor above all else for me. Does anyone else feel the same?