IE: windows only, lacks great web standards support, lacks many features you can get with other browsers like Opera and Firefox (not important if you don't use these features though). Poor rendering and JavaScript engines, compared to others. Myriad of security issues over the years (largely due to its integration with the rest of Windows).
Opera: Generally good at everything, marred to an extent by not being open source. Tons of features built-in.
Firefox: cross platform, open source, 3.5 has good rendering and JavaScript engines. Tons of add-ons. Let down by high memory usage, and being a bit 'heavy'.
Safari: webkit rendering engine, about the best there is right now. Lots of (perhaps) useful features, Apple touch to detail (Mac OS X especially). Really good speed and standard support.
Chrome: Great rendering and JavaScript engines, great performance. Open source, so will be cross platform. Really nice features that no other browsers have right now (example, each tab is its own process, so if one website crashes, it's just that site and not whole browser), and building up to have a great plugin system like Firefox.
Why is rendering and JavaScript performance important? Because more and more sites are using heavy JavaScript to behave like a traditional desktop app. They're only nice to use if they're responsive.
HTML5 is also becoming more important. Safari 4, Chrome and Firefox 3.5 is getting good support for it, including the video tag, which may be the future for Youtube and similar sites (and we can dump the awful Flash plugin for video), so all browsers should have some focus on this.