Based on my experience with Win7 and Win8, I expect that Win10 would have crawled (or not at all worked) with the slower CPU and smaller memory.
My guess would be that the Raspberry Pi 2 is, in fact, a Microsoft initiative rather than an RPi Foundation initiative - and that Microsoft have actually contributed significant funds to make this possible. Either way, that is good for the community - I don't have a problem with Microsoft financing my stuff as long as it does not come with any strings attached (which, I trust the RPi foundation to have insisted on).
Pure speculation, of course - but I'm sure we'll hear a lot more about this deal.