Type on a real keyboard. Use any website, including Ajax, Java and Flash. Use any off-the-shelf app you need. Play any video file you happen to have, without recoding. Use just about any peripheral under the sun, including USB host. Print. Enough ? There's much more though.
An IT is only a laptop/UMPC if you're willing to limit your usage of the one to the capabilities of the other. But they're definitely not the same thing.