I second what everyone has been saying and I just wanted to add that I like it when watching a video in full-screen mode, the controls automatically hide, but upon moving the mouse, the seek-bar appears at the bottom. I like that better than having to click and having a little box appear somewhere. Seeking on a little box is harder than having the entire bottom of the screen. Look at VLC player's default skin. It drives me crazy that to seek, I have to exit full-screen mode (or at least I don't know of any other way to do it).
I also like the ability to click on the "time remaining" and toggle it from time elapsed to time remaining.
I absolutely LOVE the ability to pan in The Core Media Player. Many players can fit your video to a given aspect ratio, but TCMP goes beyond that and also lets you manually adjust it (via right-click menu options or even better by simply pressing the page-up and page-down keys).
I also really like TCMP's ability to control the volume by simply scrolling the mouse wheel.
I use TCMP to watch videos (it can play just about anything), but for music, I prefer iTunes because of its organizing ability and flip-cover display. The only downside to iTunes is that it won't synch with non-ipods.
Okay, if I think of something else, I'll post it later.
