You'd have trouble doing most of those on an Iyonix as well, so you obviously have some good reasons to be using a Windows machine!
I think to be fair, you can achieve an element of some of them under RISC OS. E.g. there is an article about creating jpeg slideshows for DVD players in August's Archive. You can do programming on RISC OS, but not in C# (just as I can't do native ARM coding under Windows). You can play games, but not Unreal etc.
But certainly if you have specific Windowsy things that you want to do, you're going to have to use Windows. I think the point is really just that if you're going to make a comparison with recent PCs, you should compare to the modern RISC OS hardware too.
Yes, your USB Jetflash device sounds very similar. If it is a "Transcend JetFlash", then according to the Iyonix website, this would work with an Iyonix without any extra software ([link]). This may well work on a RiscPC too, but you'd need to buy a USB upgrade podule if you don't already have one.
In terms of software, I think RO 4 and RO 5 are fairly similar (RO 4 is better in some areas), but really it's the hardware that makes the difference with the Iyonix - largely down to the speed increase. It sounds like such a minor thing, but I certainly didn't anticipate how much more productive the extra speed would make the machine.
You'd also need USB, Viewfinder, sound recording, CD writer, flash card reader, fast IDE interface and so on to get a Risc PC up to a similar feature set.