It seems that whenever I am a lobby leader in a game of left 4 dead 2, when I start the game me and whoever happens to have joined my game end up on a server where everyone EXCEPT me has a ping > 1500. My ping is typically around 50, but isn't much use when it's unplayable for everyone else. This behaviour is consistent and happens every time I'm leader, even where I join a lobby as a guest, then someone leaves and I become leader. When we start, rubbish pings for everyone except me! Has anyone else seen this strange behaviour?
It could be you started a local server rather than joined a dedicated server, I see this happen quite often, maybe when dedicated servers are full? Only thing to do is go back to lobby and search for another server, or don't be leader