I could go on for awhile about first-party iPad apps (Maps and Photos are fantastic, and YouTube finally transforms YouTube into, well, not YouTube), and about third-party apps (ABC, Netflix, New York Times, and growing competition in Twitter app land are all keen launch-day examples), but I think that time is going to show that these apps aren't isolated cases. These apps are a part of a platform that specializes in really great, best-on-the-planet type of quality.
I think this is a consequence of a lot of things- strong design leadership from Apple, strong leadership from the "Delicious Generation" of Mac developers moving to the iPhone, and indeed, the AppStore itself for proving it's economically viable. But I think the biggest motivator is singular focus, that you're not dealing with multiple apps at a time. No window chrome to deal with, no menu bar menus, no playing second fiddle to one or two or twelve other apps open at the same time. Even if (or when) Apple introduces 3rd party multitasking, I suspect it won't impact the main one-app-at-a-time viewpoint we've had the last few years.
You play differently when you're on stage by yourself than when you're in a 300-piece marching band. Each breath you take — and each you don't take — will be scrutinized by an audience with its attention solely on you and you alone. You make sure your instrument's in good shape, your fly isn't down, your hair isn't out of place. You want them concentrating on your music rather than some inane detail that doesn't even impact your music in the first place.
The app is a soloist. There's too much competition to fuck it up. Whether or not you agree with the amount of mindshare the iPhone platform has garnered, it still is a massive player in the market today. And all of these reasons are why people are throwing their best at the platform, from huge corporate giants to small, cutting edge startups. Either way, there's some really cool shit out there now, and we're going to look back in six months and wonder how we got by without those cool apps that haven't even yet been sketched.