Recently I went to a co-worker and complained that
iTunes doesn't let you directly drag your music or audio directly to a play list. What I mean by this is that if I have "song A" that I like and want to put it with the rest of my favorite songs, I can't just add it by using the mouse from within
iTunes. He then said, sure you can, and proceeded to do it right in front of me.
I was perplexed because I had never been able to create play lists directly. For instance, I bought the Candice Millard audio book
The River of Doubt. I wanted to be able to click directly to a play list as I worked my way though the book. In order to do so, I had to browse to the files through explorer and drag them from there.

After discussing the issue at length, we finally found the culprit. I use an external drive on my network hooked which has a media center built into the drive. I have this drive mapped to my computer, as well as having full access to it.
iTunes readily acknowledges that the drive is there, and by all accounts appears to work as if the files were directly sitting on your hard drive. However, under these conditions, if you drag an audio file from a media center it just sits there. No error message, no instructions, nothing. It just doesn't work. If the file is browsed to over the mapped drive, it drags over naturally.
It makes sense that iTunes reacts this way because there's no telling what access one has to a media center share. It's basically just stream access to the files. As a user, I would have probably never figured this out if my co-worker had not thought about the scenario described above. The solution would at least be to show a message stating that the file could not be added because of the source.