For a start I won't even look at it until I have read a wide range of reviews, particularly from players whose opinion I respect i.e. not given from a biased perspective.
The only real deal breakers for me:
A clear and simple DM guide to create easy, medium and difficult challenges (but ultimately fair challenges) for PCs
Easy to run, but interesting, challenging and emblematic monsters with practical combat stat blocks.
All classes are as enjoyable to play as the others in their own way i.e. no glaringly bad classes, overpowered classes, boring classes.
PCs feel heroic not pathetic e.g. lv 1 characters not outmatched by house cats
Classes do not suck for 5 levels and then become gods, making everyone else feel pointless.
Class features available so that classes don't have to resort to using completely anti-iconic weapons in order to have something to do in combat e.g. a wizard using a crossbow.
High Level Spells do not break the game
Mechanical rules are slick, relatively simple, and easy enough to memorise so that having to refer to the rule books to adjudicate a decision does not happens on a regular basis.
And here is a real big one for me: The ability to have full on tactical combat where terrain and positioning are significant. I would be fine if this was a dial I could turn up or down as the significance of the battle varied, as long as a genuine, fully developed option still exists.