Hmm I had an iPad 1 64GB 3G that I bought with IOS 3.x on it, at the time I purchased it I could use the camera connection kit to read my USB memory sticks. Apple decided that they didn't want that, and took that ability away from my iPad 1.
Also add:
- anti-aliasing while running iPhone apps in 2x mode
- lyrics in Music
both taken away in iOS 5. The first (anti-aliasing) as a "bug" (pathetic explanation by Apple...), clearly to make people switch to the iPad 3 (also) because of the now-ugly, pixelizated (because it's not anti-aliased any more, unlike in the pre-iOS 5 days) iPhone apps.