The iPad was not a risk for early adopters. If it had flopped, the adopters would have still had an oversized iPod touch and a range of software to run, just not making good use of the extra screen area.
(The iPhone similarly was no risk - originally it was a music player and phone in one, the smartphone ability came later - the originals couldn't even multitask).
RT is a huge risk. If it flops, (looking highly likely), what use is it? No new apps. And given Microsoft's history with non x86 operating systems being dropped like hot potatoes and not having any compatibility with their replacements, being locked to an App store is a kiss of death.
Also there is the perception of non techies. They think of a computer, a laptop, a PC, Windows, Microsoft office, word and excel, as pretty much one thing. They think of Macs as expensive alternatives.
They think of tablets as cheap alternatives to do most of what they want to do on a PC, but with less hassle, (antivirus, system updates, system maintenance, software installation etc.) And they just fire up the old PC when they *really* need to do something with it.
What Microsoft have missed is the perception that tablets are an alternative to Windows. They successfully managed to prevent this happening with netbooks. (But because the form factor is different, they can't do the same again.)
A bit like Ton Hank's character in Big, they have lost track of their target market.
They are producing a class of item that would appeal to techies, but with a system that is obviously aimed at non-techies and a price that is aimed at iPad buyers.
A typical reaction the the Surface RT on here is 'I'd love one, if I could install my own OS'. Another typical reaction is 'I'd love one if I could install my own software'. Few here are happy with the App store lock down.
Non techies are quite happy with that limitation. But the history of windows puts them off. (ironically the core system of RT is probably pretty good). Why would they choose one instead of an iPad when the price is close? And those with an Android phone already know how to use the low priced alternative.
RT really is a non starter. If it ran WP8 apps it would have a chance (at least to survive as a niche product).
WP8 is not in such a bad way as RT, it will probably remain in third place for a while, until BlackBerry recovers (if Nokia managed to salvage half the Symbian market, surely BlackBerry can salvage a fair chunk of what they had.) Phones are a different enough product that the Windows smell won't be such a taint. (Though more astute potential buyer will ask themselves if the phone will still be supported in a year.)
I think Nokia has managed to retain/regain the non power user part of their users. (especially now replacing a high end Symbian phone doesn't involve a huge step back in hardware.) The power users have all gone to Samsung. Sadly for Nokia their smartphone user base had a much higher power-user proportion than the general population, hence their belly flop.
I think WP can be regarded as a success for Microsoft, in that it removed a competing OS that paid no royalties and took a chunk of it, while the bulk went to an OS that pays royalties. For Nokia of course, it is a somewhat different story. (Perhaps they would have been better off making wellies)