We developed a web based game BattleCell that uses Ajax/CSS instead of Flash for all the heavy lifting. We discover at least one new bug in the IE rendering engine every month. Our pile of IE bugs in the back room that we have to track every time we develop a new feature is testament to the dread with which we view this new hardware-based rendering engine. We know what we're doing.
Just last week, we learned that once you have a stack of enough semi-transparent layers (combination of PNGs with alpha channels coupled with DIVs with various opacity CSS settings), IE fails to render the top-most layers. This doesn't happen after 20-30 layers. This happens after 5-7 layers. At first we thought our code was faulty, until we realized that scrolling down such a page with multiple layers will cause text that was previously "invisible" to suddenly be rendered in its specified color... as we kept scrolling, the text would then disappear again. You get the idea.
Obviously, this all works flawlessly in Safari, Chrome, Opera. For IE, we get to re-architect all sorts of work-arounds --a house built on sand.