Some images don't display correctly, it leaves that x mark that we're all familiar with, and neither does graphics on pages (on some sites). Sites still think you're running IE7 (even the previous version was like that, in the fact that it didn't show the correct version number). It takes them quite a while to release another version, so if bugs are reported don't expect to see any change until the next version, and even then it may not be addressed.
As mentioned in another thread .gifs run slow (still after so many years), one page in a certain thread on a forum that I visit freezes the entire browser. One other site I visit has to be revisited in order for the fonts to display correctly. Ads lag the browser on Ustream when you're watching a recorded broadcast.
In order for you to be able to search for your favorite sites quickly in IE, you have to leave indexing enabled for all except drive c. In Firefox it's very convenient and you can quickly find your most visted sites, even in Chrome too.