Relearning To Close Windows
Jensen Harris has a post about how they tested out removing the ability to double click in the left corner of a window to close it, but that people couldn't get used to it so they're putting it back. Might I suggest that people didn't get used to it mostly because Office was the only application that didn't allow it?
If you want to really test out whether or not people could retrain themselves (and they can, even if they complain about having to) you'd need to remove the functionality from all the windows, not just a few of them. Otherwise, the other windows preserve the user's habits and it just becomes really frustrating that the interface is inconsistent.
Whether or not you should be able to close windows by double clicking the top left corner isnt' really important, what matters is that the interface behaves consistently and doesn't surprise users.

June 9th, 2006 at 8:30 pm
Yeah I agree about the consistency thing - just switching from Windows to Gnome is enough to completely baffle me. I couldn’t count how many times I’ve clicked the left-hand icon in Gnome, whose sole purpose is to leave me torn between clicking the “close” option offered to me, or reverting to what I should have done and making a reach for the X in the opposite corner. Oh the milliseconds I’ve wasted!