I will bet that you don’t even think about alt-tab as a problem, but you struggle with it every day.  You are working on a blog post while alt-tabbing to a reference article on the subject.  Suddenly an IM window pops up.  You say hi to your friend and then alt tab back to your writing, but the next time you intend to go to back to the reference article, but you end up at the IM window.  You spend a few seconds flipping through the windows until you are were you want to be, but by that time you have lost your thought.  Its a pain right?

To any of us who use more than one window at at time, this is a nearly constant problem that is embedded into just about any OS we could be using today.  What we really want is a system of switching interfaces that learns what we want to switch back and forth between, rather than simply sending us back to the last place we were.

Aza Raskin has a very lucid discussion on this problem that I wish more OS designers would read (or at least hackers with the skill to write a mod for alt-tab behavior.)

Source: Aza on Design.