@tdale
By that I mean how sandboxed everything is. iOS8 improved matters, but there's so much you can do between applications on a PC/Mac that are impossible on an iPad.
Equally, all the relatively simple things that a PC/Mac can do that I take for granted - use Subler to remux an mkv to MP4, batch rename a tonne of files, meaningful file management beyond simple directories, Automator, bunging some stuff on a usb stick, conversion of a mobi to epub, multiple desktops, install firmware on a random gadget, torrents, newsgroups, emulate Mario Galaxy at 1080p* etc etc. Honestly, I could reel of a hundred little things that are either impossible or very unintuitive on an iPad that a traditional computer can do as simply as adding 2 & 2.
Please don't misunderstand me - I've absolutely no need or desire to do any of the above things on an iPad. If I only had an iPad, a lot of these tasks I simply couldn't perform. As such, when people say an iPad is their one and only, I can only assume they have a very narrow use case, heavily reliant on cloud services and don't feel restricted by the iPad's numerous limitations. That's not a criticism, I'm happy these people are satisfied, but going iPad only would feel like losing a couple of limbs to me.
Neither am I saying these things aren't conceivably possible on a future iPad, but realistically, however far the iPad evolves, it'll always be heavily reliant on it being throughly entrenched in Apple's world/vision.
*Jailbreak or a Surface may enable such functionality, but doing some of these things on a tablet would definitely falling into the "because I can" category when a laptop/desktop sat next to it could perform the same task a million times better.