For my use, I want something as light and as slim as possible -- adding minimal bulk or weight to the tablet.
To me, the iPad2 smart covers plus the x gear rear cover are a near perfect combination for my needs.
The rear part is something like this, but from a different company:
For my original iPad, I had a one piece cover that was very similar -- a thin, form fitting shell around the back (clipped on corners, which also protected them) with a thin, hard, segmented front cover. Not as durable or solid as my current iPad2 setup, but very serviceable overall.
Something like this (this might have actually been it, in black, but it's been months since I sold my original iPad with its cover):
Since I carry my tablet(s) in a small shoulder bag, I want something that will protect the screen from scratches, or even the odd bump, but that introduces minimal "drag" on pulling it out.
I favor the "prop" characteristic (low angle) more than the "stand" (high angle), as it helps with positioning for typing.
Color, almost always black with minimal ornamentation/design.