Seeing as how I'm not playing 3.x at the moment, the only criteria I can think of that 4e needs is for it to be easier than 3.x for me to DM.
If they fail to make it run faster/smoother with less prep time on my end, I'll just stick to using Savage Worlds whenever I can get the time to play (which isn't right now as I'm working on my Masters, started a new job, dealing with other family related issues).
Oh, and if they keep the same Vancian spellcasting as before. It might not be a deal breaker, per se, but I would be very disappointed.
Now, those are the criteria for getting me not to play it. As for not buying it, there isn't anything they could do as I'll buy a copy of all three books to read at the very least.