The full sized on screen keyboard makes it alot easier to type (but see above post about my opinion about typing on osk), also the screen is sharper and obviously there's more space. Thus allowing for more things on the screen (if you run openoffice on the N900 you can tell it gets cramped from time to time, but whether that bothers you enough is individual dependent). It does feel like a oversized iPod touch rather than a tablet at first, but after you use the apps that were made for the iPad (not apps that were simply rescaled from the iPhone) it does have its own unique functionality.

Now whether it's enough to justify buying one, I don't know. For me the N900 does nearly everything I saw of the iPad so far (with the caveat of hey your using battery and therefore you may have no phone functionality if the battery goes to 0. The main thing I'd like to see is games, but I don't see a point for buying an iPad for games when I prefer Nintendo or Microsoft consoles/handhelds. I'm more likely to buy the next Nintendo portable for the gaming aspect than an iPad. But for others, the iPad fills the primary role of ebook reading, tv/films, and games.