A big chunk of it are the little things (semi-intangibles as I put it before). The edges of the T1 cover are left rough and sharp (at least mine was), which made it feel cheap. Without the need for magnets, they didn't have any sewing into the cover itself, which is a small but nice touch. The back of the cover has a nice grip, but the shape tends to remind you that you are using a device. The lip of the clasp presents another edge when reading one-handed that bugged me. The front actually had something to stay put on the 650, and the T1 has nothing so the cover tends to flop a bit.
I'm not gonna say they did a bad job with what they had. It's better than the Nook covers I've tried with the STR. Hell, I couldn't even read with the Nook one-handed in the cover with my left hand because of the flimsy clasp they have (could with the right). The materials are close enough to recapture most of the style, and important for most folk, it does the job. But at the end it isn't quite the same thing.
I know the things that bug me are subjective, but I can't just ignore them. I only ever used the Nook STR without a cover (even though I tried) because of these harder to quantify issues with the covers.