Not GP, but: Realplayer compressed everything to hell, the quality was absolutely atrocious. I believe it was buggy as well.

Quicktime was a behemoth that took ages to launch. To speed things up, it liked to auto-load and be active in the system tray, slowing system start down even further and taking up precious ram on the off chance that you might want to watch a quicktime video. It also liked to register itself as the video player of choice for other formats, because why would you use a decent player if you can use a shitty one that was made by Apple? Fuck quicktime.