One of the major changes in Internet Explorer 8 is that it adheres much more strictly to web standards than past IE releases. That's a welcome and sensible move, but it has resulted in many sites not rendering well, since they have been built to work with the slightly skewed design principles of earlier IE releases. The second beta of IE8 handled this with a 'Compatibility View' button, but Microsoft's internal research suggested that this wasn't quite enough, as a post on its IEBlog explains:
We saw from the telemetry data that IE8 Beta 2 users still have to use Compatibility View a lot. Looking at our instrumentation, there were high-volume sites like facebook.com, myspace.com, bbc.co.uk, and cnn.com with pages that weren't working for end-users with IE's new standards compliant default. We could also see from our instrumentation that not all IE8 visitors to those sites were clicking the Compatibility View button. So, large groups of people were having a less than great experience because they weren't aware of the manual steps required to make certain sites work.
Microsoft's latest solution is to allow users to opt-in to an automatically updated list of popular sites that need compatibility view and have those sites rendered using the older IE7 approach without requiring manual intervention. That feature will be added to the next beta of IE8, due early in 2009 (there's a similar feature already in Opera). This seems like a pretty good interim solution to me; share your take in the comments.