One of my favorite things about OS X is the way it acts when you change a filename. In OS X the file name is all that’s highlighted so you don’t have to worry about changing or re-typing the file extension.
This may seem like it’s a small detail but when you rename files on a regular basis mis-typing and having to put in periods and remember what the file extension was can add up to a lot of time.
While this may have been fixed in Windows 7 (I don’t know, I haven’t used it), I know my work machine is still XP (and probably will be for a while). This means that every day this behavior affects the way I do my work and can quickly go from “minor annoyance” to “seriously, are you KIDDING?”
And a quick post-script: I know I can disable showing the filename extension so it won’t be changed but that only highlights another issue: the default Windows XP file renaming behaviors are broken.