You're not imagining the drop in interior quality. I also recently sold my X3 3.0si and its interior was the same material as the E90 3 series. The X1 interior materials have high hard plastic content, really to the point that it feels in many ways like a Japanese vehicle of substantially less $$. Another observation is that the cargo cover is not even close to the high quality of the E83's retracting blind. The X1's cargo cover reminds me of my '75 Scirocco's. Chintzy plastic and I reckon pretty easy to break if mistakenly stepped on while off the vehicle.
BTW, I sat in a new F30 328i, and while more stylish than an E90, the material quality drop was similar. More hard plastic surfaces along all four doors.
Oh well, the focus is on the goodness - great motor selections, RWD or X-Drive. Decent dash arrangement. And drives well.