I probably will not buy it, but might eventually
At present I have drifted a bit from typical d20. I have tried True 20 and am considering Iron Heroes. I like many aspects of the d20 mechanics, so if the system changes significantly I likely will not buy it. If it changes only slightly - well, I still have the SRD, and I can make some adjustments as necessary to follow the pattern of changes.
Basically, if it doesn't change too much then I see no point in not continuing on with what I currently have. If it changes too much then I will have to see it in play and read quite a few reviews - and likely skim the PHB and DMG in the local B&N - before I consider putting money down for it.
The easiest way for me to determine if I like it or not would be to read the rules for it online in the equivalent of an SRD - even if it only included a few example spells, conditions, etc rather than the whole meat of the system, but current rumors seem to suggest it will not have an SRD. :\
So, for now I will wait. I cannot say with any certainty that I will ever buy a single book for it, but I am willing to admit that my buying such could eventually be a possibility.
As for guesses as to what it will be like, I notice the warlock is popular, as is the Bo9S, so I'm guessing this will influence the system structure of the new system. Minis will likely become an even more core aspect of the system, to force players and DMs to buy them and thus generate revenue. Perhaps the SW Saga edition is a testing ground, seeing what the players like / do not like of some new ideas before the final decisions are made on what the new 4e system will be like. Beyond that, I have no guesses as to what it will be like. So we'll see.