It's quite true that Apple will probably try and milk us for $129 (and whatever the hell it'll translate to in Sterling these days), but I can't believe they'll sell 10.6 on Grand Central, OpenCL and Exchange alone (along with the 64-bit kernel).
Some users will benefit from these features, especially Exchange, but at present I don't see why I should purchase it. I won't benefit from OpenCL with my early 08 Macbook, and the other features are nice but not stuff that will make me rush out and get it.
Had I been a Mac user when Leopard came out, Time Machine would've been the killer feature that made me buy it (indeed, it was one of the main reasons that I Switched). I'm quite happy with it now that 10.5.6 is out; why should I upgrade when it does what I want just fine? Snow Leopard needs something if not 'killer', then near enough to incentivise me. For some that'll be Exchange, but not me.
Not saying that's the right way of thinking about it, since under the hood stuff is good and kudos to Apple for doing it; but that's just the way it is.