Paul Thurott is an unhappy camper - so much so, that he's calling for a boycott of IE7. Here's a summary of his points:

Microsoft blames backward-compatibility problems for the stalemate over true Web standards compatibility. Put succinctly, the company has gone its own way for so long and now has to support so many developers who use nonstandard Web technologies that it will be impossible to make IE Web-standards-compliant without breaking half the commercial Web sites on the planet. Furthermore, by halting all IE development for several years before reconstituting the IE team to create IE 7.0, Microsoft has set back Web development by an immeasurable amount of time.

IE is likely a large, ugly body of C++. I say ugly because of all that "does this bad stuff work" code in there. I can't imagine how hard that would be to pick up after 5 years of being stockpiled.