A file system is the biggest one. Having my home folder mirrored on my iPad.
You can use a remote PC style function to access your mac/pc virtually. Leave your mac at home online, access it virtually using the iPad. All your files, folders, apps, emails, all accessible. You wont need to re-buy you apps or double up on anything.
Having files sync automatically on my iPad and iMac (so I can work on a doc on my Mac and then continue doing so on my iPad on the bus).
iCloud
Numbers was too awkward to use - so much quicker on my Air.
Microsoft Office will be out on iPad soon
An SD card slot.
Why? Ok, camera connection kit, iCloud, bluetooth, wifi transfer, dropbox.... I admit as SD card would be useful but I never found it to be critical or without an equally useful work around
The ability to stop the iPad telling sites its a mobile device and therefore loading mobile versions of websites.
Set it to present itself as a desktop device in settings. I know the iCab broswer lets you do that. You get desktop editions of all webpages
More storage. Even the classic iPod has 100GB more than the iPad.
For what? Do you absolutely need 100GB of offline media at ALL times? Stream movies from PCs in 1080p or use cloud services. Actually i bought to ipad as a movie viewing device but I hardly keep anything more than one or two movies at a time on it.
A better GUI - I still find the tiled app display very counter-intuitive.
Whats wrong with it? The only problem I have is that after 5yrs it would be nice to see something different but its still the most elegant design. Widgets are coming along but have you seen the jerk-o-thon that is Honeycombe?
I haven't used an iPad for a few years and I haven't used the iPad, but I know it hasn't changed much from the iPad.
Apple have loosened up a lot since the iPad was launched. A lot of media players, browser and cloud services have improved its usability a lot. There are hundreds of thousands of dedicated apps that improve it further.