I got really excited about this yesterday, but there do seem to be a few limitations which make it a little less exciting:
1) What was shown was a glorified power point presentation of how they expect it to work, their first real attempt at doing this 'on hardware' will be at a demo (today? tomorrow?). The reason for this is that it requires 'new hardware' to work. This means that all current devices will not get this functionality, which is a little sad.
2) We do not know what hardware this will require. The assumption is that it requries HDMI, which is somehting that current devices do not have. We have video out via USB... but my guess is that ths is not good enough performance to have a good quality monitor connection. Many WP devices also have WiDi capabilities, but perhaps they think that the latency issues of WiDi make is less useful for productivity work (though it works just fine for media). This means that new devices will either have a propriatary port that can do charging and HDMI... or we will see a USB-c connector on the new flagships
BTW: we will not see a new flagship device at Build. What we will see is dedicated demo hardware. It may have the specs of the next gen device... but likely in a big bulkey, ugly housing.
3) No x86 apps. All windows phones to this point have run on SnapDragon processors, and as good as those are becoming there isn't a SD chip on earth that can properly emulate x86 in real time. This means only the continuum sensitive ARM apps will work. I am sure this will expand with time, but right now this is pretty much just the new Office and a handfull of 'universal apps'... mostly made by Microsoft.
In the future we could see an x86 Atom based phone, and the opening up of x86 apps for phone. But again, they are going to be limited to continuum mode only, or will only be Universal apps that have scaleable UIs. Plus ultra-mobile x86 is very limited hardware and will not run many traditional x86 applications well in the first place, so I am not sure that moving to x86 on phone is exactly going to bring the expierence people hope for.
4) This is still an incomplete feature. The true future is the demo that Ubuntu Phone showed off a while back (has it been 3 years already?) where the phone could not only attach to alternate input and output hardware, but could also take over processing hardware as well. With that demo you could attach the device to a PC and you would have PC hardware do the heavy lifting for big games or productivity content while the phone itself was able to charge. This win10 technology does not go quite that far... but it needs to have that ability in the near future. And it needs to do it through the network (so that 3rd party manufacturers can still sell PCs, and so MS can sell Azure based services to consumers), and it needs to work in a multi-user enviornment where 3-4 mobile devices can send rendering tasks off to the same home server concurrently.
Anywho, outside of that, this looks fantastic, and I thought that some of this feature set was coming with WP8.1 and I was sad that it was not ready yet. Very glad to see it happen, and very glad that the laptop I have will very likely be my last laptop.