Then there’s the issue of legacy HTML/CSS support. So much of the Web has been tweaked for IE 6.x compatibility that even Microsoft's own attempts to implement a more standards-compliant browser engine in IE8 have met with disastrous results. For me, the situation so bad that when I do find myself using IE 8 (typically, to view a site that causes my copy of the Chrome 2.x beta release to blow up), I end up configuring IE 8's compatibility mode as the default viewing option since the browser's native rendering mode breaks practically every site I visit.
Finally, there's the matter of third-party developers using IE's rendering engine with their own applications. A good example would be a program that includes a help file in HTML format and then uses a custom form to display an embedded Web browser object to host the file. This embedded object is invariably an ActiveX container for the IE engine that's installed with Windows, so any attempt to remove IE from the OS -- or to radically change its core underpinnings -- will need to account for applications that rely on the existence of an accessible, programmable IE object model.
Of course, all of the above is old hat for Microsoft, a company whose status as global software leader too often makes it a victim of its own success. I, for one, look forward to the possibility of a clean break with IE's creaky old rendering engine. But I hope the company pays more care and attention to preserving legacy compatibility than it did with some of its more recent OS efforts.