Microsoft finally has a browser that can actually play with the big boys. Since we've covered IE9 extensively already, I'm not going to rehash all the details, but here's a breakdown of the most important additions.
First up, Microsoft is the first browser to ship with hardware-accelerated rendering. The GPU is put to work through the use of Direct2D for drawing, and the new text renderer in Vista and 7, DirectWrite, takes care of text. This last thing causes what I believe to be a major issue: text rendering is absolutely horrendous on many websites. DirectWrite produces fonts that are shape-accurate - which is the way Mac OS X does it. This looks fine if the entire operating system uses it, but when only one application uses it, it stands out like an eye-sore. Windows' ClearType renders text in such a way that they are sharp - shape be damned. Mixing the two is sloppy.
IE9 might as well dispense rainbows and unicorns, but the text rendering is a definite deal breaker for me.
Another major consequence of the hardware acceleration is that Internet Explorer 9 doesn't run on Windows XP. I consider this to be a plus point - I find Windows XP intolerable and it needs to die - but those of you still clinging on to that piece of junk won't be able to use IE9.
Of course, one of the major points of interest was support for modern web standards, an area where Microsoft has been lagging behind for almost a decade now. The details here can be found in the developers' guide for Internet Explorer 9, which details all the new things on this front.
There's loads of other nice tidbits as well, like the ability to pin sites to the Windows 7 taskbar where website developers can even create jumplists for their sites (pin Facebook to your Windows 7 taskbar and you'll see). The interface itself of course follows the trend set by Chrome of focussing on the content instead of the UI, and that's a good thing.
You can get Internet Explorer 9 for Vista and 7, so if you feel inclined to do so, give it a shot.