Really? I felt like my 900 was built very well. The only point I'd make on my 920, aside from the obvious technical advances, is that the shape of it fits in the hand more comfortably and the curved glass on the screen looks nicer. Aside from that I'd say the build quality is equal. It certainly does not make my retired 900 look like a "cheap toy" by any stretch of the imagination.
Well I am not saying the Lumia 900 was poorly built (Minus that stupid camera chrome) but compared to the 920 it feels like it now. The side buttons in their short time all got slightly scuffed and they were a bit loose. The screen was just there and that white luminous buttons were ok but the lighting was uneven and it was a weird white when they are off.
Now lets look at the 920
I mean the buttons on the Lumia 920 are tight and not scratch prone, same for the camera plate. The charging port likes there is ceramic around the edge of that (unlike the 900's simply open plastic hole), the screen curves nicely into the case; and the buttons have a nice mirror finish when the backlight is off and the backlight looks so much more uniform now.
Then you take hold of your Lumia 900 again and it feels like there was no attention to detail vs the 920
Well thats how I feel anyways. I just took it out again to be sure and the buttons rattled a little even just taking it out of the box