The main difference is that opera can not run active-x since it is a proprietary ms tech. This means that software can't autoinstall as easily when surfing with Opera, so instead of getting that install box pop up, you have to press a link link to install quicktime, flash etc.
It also means that you are pretty much secure from getting spyware and viruses installed over the web, so it's a good thing. Opera also adheres to most web standards, while IE tends not to (one example is CSS, but there are many others)
Other than that Opera has nifty features like tabbed browsing, easy privacy control, pop up blocker, mouse gestures etc.
You may also want to check out mozilla which has many of the same features.
My new years resolution is 1280 x 1024