Having an iPad 3, here are some things Apple could improve on:
1) Lighter. It's a little too heavy.
2) Cooler. It runs too hot. - fixed in iPad 4 I believe
3) Better edges. The current rounded edges are a pain for holding, plugging in, etc. The surface looks much better in this regards.
4) Volume control - move it away from the power button. I hate turning the device off when I want to change the volume. I also hate how it mutes the volume when you turn the volume down.
As far as the OS, it's good for what the tablet is used for - web browser, mail, netflix, some games.
Having used Windows 8 for a bit, it appears it would be really nice on a tablet. It's not so nice on a dual screen setup. Actually reverted back to Windows 7 due to usability. Microsoft did improve a lot of the core in Windows 8, but the new UI can be crippling for advanced users.