Shawn brings up some great points and I agree with most of them. One thing that I don’t quite agree with though is the implied motivation for Apple to launch the App Store. Many think Apple launched it for profits, or to simplify their software — I don’t think I agree with those notions.
I think Apple launched the Mac App Store because they felt forced to open one. Perhaps that is silly sounding to most — they are a huge company and rarely listen to any one, so why would they feel forced to do anything? My guess is that if they truly did this as a simplification of the OS they would have waited until 10.7 — giving everyone a clean breaking point for making the transition. ((Not to mention a great bit of marketing material to push a new OS update.))
Apple felt forced because they wanted to keep their fantastic iOS developers in the Apple family.
They wanted to get them on the Mac.
Apple reached a fork in the road: they could have gambled on iOS developers being willing to develop for the Mac when they release 10.7; instead they chose a safer path of launching now, at the very moment interest in such a distribution channel was at its peak.
Sure Apple wants users to find great software, yes they want a cut of developers profits — all of that though could have waited until 10.7.
After all Apple isn’t hurting for cash right now and their users have been pretty happy without a Mac App Store.