Simple reason: Gnome does not support drag-and-drop in combination with alt+tab (see bug tracker), but there might be hope with the upcoming Gnome 3.

[Edit 2011-06-14: Indeed, drag-and-drop + alt-tab works since since Ubuntu 11.04 (did not try with 10.10). One major difference remains: Do you need a lot of configuration and customization options? And are you will to accept complex, maybe not that self-explaining menu structures for that? If yes, KDE is your choice, otherwise Gnome might make your life easier. See also this page for more details and screenshots.]

In more detail: using drag-and-drop together with alt+tab key combination allows to work very efficiently. For example, while ordering my photos, I want to work on one of them — as I do this regularly, Gimp is opened already, but in the background — so what I do using KDE or MS Windows is, I grab the picture, switch to Gimp by using alt+tab and immediately drop the picture without moving the mouse at all — I am quite confident that this is the fastest way of opening a picture for editing. Some people advice to set Gimp as default application to open JPEGs, but I am not always editing pictures, most of the times I just want to view them.

Using “Open With” on a JPEG file for sure is the common approach — but let’s compare it:

• Drag-and-drop & alt+tab
  • Actions to be performed: mouse down + key down + key up + mouse up
  • In total: four fast steps
• “Open With” solution (holding mouse down as improvement)
  • Actions to be performed:mouse down + moving to “Open With” + wait for sub-menu to open + move to sub-menu + mouse up
  • In total: two fast, three slow steps