I’ve already noticed one particularly important instance where IE8′s Compatibility View is essential: the Plesk 8.x management console. If you do not turn on IE8′s Compatibility View, Plesk won’t work.
If there is a moral lesson to learn here, it was to avoid browser specific code at [nearly] all costs. In other words, make it a point to NEVER detect for different browsers and put the onus on the browser creators to render standards compliant pages correctly.
Those developers who thought they were doing the right thing by detecting for IE and having their websites behave, serve content and run JS code differently are now faced with the prospect of seeing their pages break on IE8. Tsk, tsk, tsk.
The good news is that going forward, you should not have to worry about browser specific coding anymore (or at least worry much much less) which should have always been the case from the beginning anyway.