From a developer standpoint, IE can be a nightmare.
Unlike other browsers, IE does not follow CSS standards (2 nor 3). They have their own filters that can sometimes cause rendering issues in other browsers. Then there's
VB and ActiveX controls. Fun!
At my shop we design sites for mozilla compatibility and follow the DOM strictly. We're trying to go for HTML 5 strict but unfortunately the majority of our user base is using older Versions of FF (<3). We see about 42% FF, 28 IE, and the rest split between chrome, safari, misc.