I've found this thread really interesting because I've found myself questioning my own tablet/computer demands over the past week.
My iPad Air contract is expiring. Like many, I was disappointed that there was no Air 3 and I think the Pro is too big (it would require buying a whole new raft of handbags for a start...). I was unwilling to go to the Air 2 because it's 'last year's tech' and by the time the standard 2 year contract is up it would be 3 years old. So ... I took advantage of a great contract deal on the Surface 3. I spent years on Windows, I know it extremely well, I knew I could make it do what I wanted and I liked the idea of a tablet that could be a laptop (or vice versa, depending on your priorities). It would be great! A full file system, an OS capable of running 'proper' apps ...
It hasn't worked out that way. My main computer is a 2013 rMPB and I have an iPhone. Even my work computer is a 2012 rMBP. The Surface stuck out like a sore thumb. I liked some elements of it (the hardware, some parts of the tablet interface, OneNote, handwriting recognition) but before long I was reminded of everything that has ever annoyed me about Windows. The scaling of desktop apps wasn't great, even at over 200%. I could get by in Explorer but almost everything else was nearly unusable for me on that tiny screen. The 'touch' apps are lacking and even in tablet mode there's little of the touch-screen precision that makes the iPad such a pleasure to use. I know there's the pen (and I do have a stylus) but I personally don't think that a tablet should need a keyboard or pen to function well. The Surface really does need either or both, as many reviews will testify. In essence, it's a fabulous device for specific use cases but does not perform brilliantly either as a laptop or a tablet.
By contrast, most iPad apps are designed to be highly functional through touch and the interface scales incredibly well. Yes, editing video, massive images or huge and complex spreadsheets may be difficult if not impossible—but isn't the same also true of any low powered computer, including the 'proper' computer that is the Surface 3? Even my rMBP has balked at some of the very large images I've occasionally had to juggle in work.
Most of the traditional objections to the iPad-as-computer have been rendered nearly void by iOS 9. We now have split screen, even three screens if you consider 2 split apps plus PIP. iCloud Drive offers a smidgeon of file control. We can upload to Safari and download to third party apps. We can forward attachments of files other than (and in addition to) pictures. We can draw, paint and edit images to a reasonably high standard in apps such as Pixelmator and Procreate (in fact, I personally prefer image editing on the iPad these days; it feels more natural). There's Microsoft Office for traditional productivity needs. Scrivener is hopefully coming soon and in any case the iPad is well supplied in the writing line. It would be nice to have more flexibility in terms of the OS but in terms of actual apps ... well, we're very fortunate. The iPad Pro will hopefully spur developers to new heights and we may see apps that push the envelope much further for both the Pro and the Air 2. A computer is a tool and any computer, any OS, is only as good as the apps you run on it.
The iPad even a level of video editing and granted, iMovie is not Premiere Pro or Final Cut—but it's as good as most of the low-cost, low-featured video editors that the vast majority will use on low to mid-range Windows laptops for churning out YouTube videos. General media for consumption can increasingly be handled in the cloud; my own choices for music are Google Play plus Deezer—I don't have any actual media files on my iOS devices at all—and StreamNation for video.
Specialists (note I do not say 'professionals' because it is insulting to imply that one particular use-case is 'professional' and another by implication is not, even though both are used for professional purposes) and power users will always require more power and flexibility than the tablet—arguably any tablet—can provide, but iOS has now matured sufficiently that for the vast bulk of the population the iPad can be their main computer. Even iTunes has become largely optional with backup in iCloud. In two years I can count on one hand the number of times I've had to link an iOS device to iTunes and even then it was more often than not a precaution. I think we'll see that umbilical cord being cut completely sooner rather than later. In many ways the post-pc era is already here. We just haven't completely realised it.
And whoops, I seem to have written a mini-essay. I do apologise.