For me, here are some quick thoughts off the top of my head:
1) Original designed case. Not the boring round case.
2) I like cases w/ different finishing combined - polished, brushed, PVD'ed, etc. But it all has to be put together harmonious!
3) Strong features of the case which give the case it's recognition
4) Comfort & wearability - are there awesome movable lugs like on my Bulgari P10 Diagono X-Pro?
5) Size preference - I like larger sized cases.
Cheers,
Anthony