IE is a pain in the arse to develop for. It's not very standards compliant and you usually need to use "hacks" to make things work. It's slow. It's full of security holes (at least IE6 was, IE7 is just somewhat annoying). It usually uses more memory than firefox or opera. They didn't introduce tabs (IE7) until loooooong after opera and firefox. I've been using tabs in Opera for about 10 years now and IE7 only just came out with them. It's targeted by most worms etc ie. installing themselves as IE "helpers" so you get random ads popping up, unwanted toolbars etc etc.
I haven't used IE7 much really but generall when IE crashes you lose all your windows. Opera comes with a "recovery" thing so when it crashes it can reopen all your windows/tabs where you were. (you can also save and load "sessions" etc). Firefox also has a similar plugin.
Let me know if you need more reasons