Why are people expecting NX to be more powerful than this?
Nintendo doesn't make the most powerful consoles, at least not for many years. If the NX is on par with the status quo of XB1 and PS4 after 2 years, that will actually be an aggressive move on Nintendo's part.
Sure, when 1080p TV's are the norm 900p is not ideal. Yes, it's disappointing that the current generation of consoles can't easily do 1080p at optimal frame rates for all games (not even PS4). Yet the games are still damn good looking and fun, and that's enough for most people -- including myself.
Also, obviously NX will not be limited to 900p. WiiU has some really pretty 1080p games (i.e. Smash), and so will NX.
Reality is that consoles will probably continue to be behind the curve in display resolution. Next gen will easily do 1080p at 60fps, but by then 4k TV's may be as common as 1080p are now, and people will be mad that consoles can't output 4k at high frame rates. And after that, 8k will come etc.