Well, for one thing its format support is conservative. Like most people who haven't lived in the Mac ecosystem their whole computing lives, I have my music collection in a variety of formats- mp3, ogg, flac, etc. I would like very much to have a media player which didn't ask me to bend to its will. This of course is not an issue for most people, but its a sore sticking point for people who try to switch from Linux. In Linux, if you find a file in an obscure format, you can always find a gstreamer plugin for it and totem(or anything else) will play it without losing a step.

I myself tried to switch, but gave up after about a week. One of the main reasons was the iMac's boot time. After having used Fedora and Ubuntu, I found it unbearably slow. Was my experience just an anomaly or is this the general case?