Quake 4
Not a great deal useful to say about this considering Raven screwed it up right from the start. Auto-downloading new files from servers failed miserably leaving newcomers and pros clueless as to what files they needed to join. Searching around for 30 minutes to find a server you could actually join killed off most of the new crowd. Now what's left? Previous quake fans and overpracticed regulars making it very difficult for any casual players to break onto the scene.
It is not all doom and gloom as id software try to make the most of a bad situation. At the time of writing there has been a few patch releases and continual development by respected mod teams, such as q4max and rocket arena 4.