The XSi in my opinion 'feels' cheap because it is so light; I want my DSLR to feel like I am holding onto something substantial in my hands. The XSi is very light, I'm not saying it's a bad thing, it's just my preference.
You bring up an excellent point about ergonomics. It's important how a camera feels in your hands and you should always check out the ergonomics before buying: is it comfortable, balanced, are the buttons and dials well-placed?
In my case, the smaller size and lightweight of the XSi is exactly why I bought it. I tried holding larger cameras and they were just too heavy and bulky for me.
With cameras, as with other things, the more you pay the more features you get, eg: metal casing vs. high impact plastic, weather sealing vs none; faster burst speed, etc.
But lack of high end features doesn't necessarily mean lack of image quality. Many of today's entry level DSLRs, such as the XSi, give outstanding image quality and have features only dreamed of just a few years ago.