Remember it'll have native Exchange support; that's reason enough for some people.
At first I was kind of skeptical too, but having thought about it; if it makes more efficient use of my Blackbook (ok, so OpenCL won't do much for me but never mind) then I'm all for paying for it. Could let me eek out another 6 months on the same hardware before upgrading.
People valuing features over performance is one of the reasons Windows has ended up where it is today; bloated, chock full of legacy support most Home users don't need. Thankfully Apple isn't going down that road.
I still reckon there could be another killer feature they're just keeping secret though? Just to sweeten the deal, since it's looking like Windows 7 will hit the same Quarter as SL?