Standards, speed, stability, ease of use, features would all seem to be important.
IE still has a bad reputation because Microsoft in the past decided to try to make their own ideas the "industry standard", largely ignoring the official standards, and because there was a period when IE development practically stopped. Versions after IE 6 got progressively better so there's no reason why you have to change if you are happy with it (although if you aren't using the latest version you should really consider upgrading).
That said, I suspect that IE is still back of the pack overall.
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