The problem with Launch Pad is that it's too geared toward touch-screens, which no machine running OS X currently has! There's a fundamental disconnect between the Magic Touchpad and the screen, same as with the touchpad for a laptop and it's screen, same as for a keyboard and mouse and the screen. This means that Launch Pad is essential a design with no problem to solve.
If iMacs or the laptops were to start coming out with touch-screens (I'd love a touch screen iMac) then Launch Pad could be great, but even so, for proper laptop and desktop use a touch-screen is more of a hindrance and novelty feature.
Launch Pad bound to a key-press is interesting, but it doesn't really solve any problem that the Dock can't, and the Dock does it better. I have my dock with hiding and magnify on, and my applications all sensibly grouped, and it's just so easy to use.
If Launch Pad could extend this into a full 2d (rather than purely horizontal) space, as a sort of mash-up of Dock + Stacks, then it could be a great usability feature or even replacement for the dock, but it would require a ton more work, and it isn't even clear if that's Apple's intended direction. It just seems like they've taken the menu from the iOS and said "hey let's slap it in OS X and call it a feature", but in reality it's not a very good one.
It just seems like a bit of a lost opportunity, it even lacks other potential benefits like App Store integration, which could have been great; see and use all your installed apps, with quick access to new apps or updates, and all in a managed interface for handling easy install/uninstall. Instead we're getting two separate entities, neither of which fully feel like they belong.
Whew, that turned into a bit of an essay! Not a complaint against Lion, as I fully intend to get it for the many other features it has, Launch Pad just isn't one of the ones I see myself ever actually using in its current form.
Launchpad would need two things for me to even consider using it as a replacement for the Dock:
-The ability to remove apps other than those bought from the Mac App Store. Right now, it's cluttered with a bunch of small junk apps and installers that come with the Adobe CS and Microsoft Office (like Office's Setup Assistant, among others).
-The ability to highlight apps quickly by typing their name and launch them by hitting Enter. This (for me) would make Launchpad much more useful than it is now and at least partly make up for the quick accessibility of the Dock.
I liked when stacks came in but better when stacks improved to allow multiple view styles including using grid view to drill down through folders. To me, this is better than LP in a way. However, LP let's you group/categorize apps that the application stack does not. Though I hear it is quite buggy still in what it will let you delete and reverting after restarts, etc.