To me it’s 3 basic things:
1. Better text input method that does NOT require putting the iPad down, is as fast and accurate as a physical QWERTY keyboard, does not take up half the screen real estate, and is not audible
2. Better apps that fully utilize iOS features and are as robust as their desktop counterparts
3. Versioned backups and easy file retrieval
#1
To me, the point of a tablet is that you can use it while holding it and doing so comfortably. If you set it down and prop it up, and add a keyboard and mouse, you just end up with a laptop again, but not as optimized. I already have a laptop that is designed from the ground up for when I’m at a table. You can’t beat it at its own game. This would allow the iPad to be a replacement work device for me (*with some caveats) by embracing the tablet form factor, diminishing the need to adopt another form factor. That said, this a very tall order. Handwriting recognition could work if implemented well and system-wide, except it would require a pencil.
#2
Echoing previous posters, most apps are just not as fully featured as desktop applications. I don’t know the whole story as to why but I don’t think it’s because of hardware or software limitations set by Apple for developers as much as financial—and this does need to be resolved. And I think most of my file system woes would be satisfactorily solved if apps would just properly implement share sheets.
#3
iOS devices can be automatically backed up to iCloud, but it’s paid subscription if you have over 5 GB worth of data (which I do), and most critically it doesn’t have versioning. Or iOS devices can be backed up to iTunes manually over a cable, and if your iTunes is backed up to time machine then you have versioning. But manually backing up is a chore. And neither option supports an easy way to retrieve old files individually. You have to restore the entire device. I need a way to restore files, a la time machine, and an option to automatically wirelessly back up to iTunes when on the same network and plugged into power.
*I don’t see ipads ever fully replacing desktop OS computers because of a couple reasons. At least for now there still needs to be some sort of desktop computer to handle backups and things of that nature. Whether you trust that computer to belong to some company in a server farm somewhere or whether it belongs to you in your home, it needs to exist.
And then there are inherent things about the iPad, namely the requirement that it be a hand held device, that makes it unable to compete with bigger, more powerful laptop workstations which don’t have that limitation. Not an issue for most probably, but for some (including me). Also with touch UI, not as many controls and as much information can be displayed on the screen as with pointer UI. But maybe an interesting idea would be for productivity/work apps to have ‘pencil UI mode’ where the UI becomes more like MacOS applications, showing a more expansive UI, but to be used with a pencil (still no keyboard, because an iPad should be held in my opinion).