Some folks at think secret were taking the position that this can't replace Photoshop and Acrobat Pro. While I agree, I would rather fire up preview and do some quick things rather than Photoshop.
Also, Preview is still lightweight, all the additional tools are hidden unless you want them. And you don't have to use any of the stuff, you can still preview just like you do today.
I OFTEN use photoshop to scale images ready for web quality but it's so annoying because it takes so long to load. If preview can do basic things like resize, crop and change image format I'll be very happy