I think you actually just nailed the three biggies. The reason is that each of the three works in the same way: it gives you a list of "things to get done," and in two of the three cases that list goes on for a long ways. (Email does too, I suppose, if you really use it extensively.) The only other thing I think fits into this case would be social networking.
It's why I was interested in that new Times application for the Mac, the one that turned RSS reading into a sort of newspaper. When you have that format, then you're expecting to look at something only to have it come to an end. It's a different mindset. At the same time, though, it doesn't solve the problem entirely, problem being: the Internet moves constantly, not only at set intervals.
I wonder: is there a way of limiting that without alienating people? Of setting a precise limit to how much can be done during any given time, then ending things up? It would make it a lot easier to avoid sites. At the same time, though, it would slow things down to a crawl compared to the speed at which things move now.