Not to diminish your post, but this is an issue that was widely reported when IE7 was introduced. I don't know that most folks understood the implications of it, though. Just to add a piece, there is another part to the story with regards to any sites that use 'active-x' controls to present content, like flash, media, and whatnot. This is the piece that may show blank areas, or does not immediately activate controls to play embedded media. You'll get the message, 'Click To Activate This Control'. That's due to another issue with MS and lawsuits, and copyright stuff. So, the point is when you manage web sites, it means the job is never ending to keep up with the late-breaking changes - not only in DNN but in so many other areas.
One thing about it, though, is if you're good at it, you can sell that point to your clients - you manage things so they don't have to do it. The other issue is - you really have to stay on top of things, it's not good enough just to develop a cool new site for someone, and then leave it alone.
If anyone wants more on the topics, just 'google' it, there's ton's of info...