If I've got one objective that I'm trying to achieve, I might need to meet several sub-objectives along the way.
So if I've got visual studio open and I'm working on my main feature, then I notice I need to get something out of a folder, I'll open an explorer window and navigate to that folder. While I'm there, I realise I need to do something later, so I'll make a note of it in notepad. A colleague then asks me to fix something in another program, so I'll open another instance of visual studio, and maybe firefox to look up the issue in the bug database, and another explorer window to find the data.
Pre-windows 7, my taskbar would now have 6 items on it, in the order: visual studio 1, 1 explorer window, notepad, visual studio 2, firefox, and another explorer window.
The items on the taskbar were arranged in a similar way to how they were ordered in my mind. As I complete sub tasks, I can close the programs one by one, from latest, to earliest, until I've finished my main task. That was good.
Now, windows 7 groups similar items together. Which means the mental associations between the second visual studio and the explorer window (for example) are no longer matched by what's on screen, which means I have to spend mental effort connecting the various items whenever I try to work out what I was doing. And since lots of my work involves putting my mind through a mangler (figuratively), any extra mental effort that I have to put in that I didn't used to have to do, isn't appreciated :-(
Luckily, there's (at least) one app which "fixes" this for me. Windows 7 Taskbar Tweaker lets me change the behaviour so that buttons are no longer grouped. Back to full productivity again! :-)