The real problem isn't that IE8 is more broken than IE7, the problem is that it's less broken.
You see, the reason that webpage designers hated IE6 and IE7 was that they had several bugs that caused pages to render incorrectly (images loading in the wrong place on the page, scrollbars not working with the "standard" code, etc), so web page designers would actually put in little hacks in their webpages that recognized which browser you were using, and would adjust the layout and functions of their page to correct these bugs (so, when the page finds out it's being loaded on IE, it automatically moves something 17 pixels or so to the left to compensate for the bug in IE that causes them to appear 17 pixels to the right of where they should be, etc). Yes, that's right, thanks to IE's predominance in the market (because Microsoft's consumer base wouldn't bother to try anything else no matter how much better it worked, and M$ felt it was everyone else's job to conform to their specs instead of vice versa), developers actually had to make two separate versions of their webpages, one hideously broken in order to make IE render it properly.
Then IE8 comes out, and patches these bugs. However, pages can recognize which browser you're loading in, but it can't recognize which VERSION of the browser it is, so when it recognizes IE it still loads the IE7-compatible version, which is broken when viewed in any browser that works properly.
(for a more articulate explanation, just read this [http://www.joelonsoftware.com/items/2008/03/17.html])