Pretty much everything that Anomaly said.
People that attack (not flag) with... scouts. The true mark of a lagger that thinks he's a pro when he takes down a Juggernaut after taking 0 damage from several rockets.
The "bouncer" ship; used for firing bouncing bombs blindly, hoping they hit something. No skill required, whatsoever.
People that repeatedly re-enter the zone after being forced into spectator mode for extreme lag. You'd think they would get the message after a while.
The ridiculously high lag limits.
People that fire thousands of rockets into your base through the "scout holes" at the north/south. Cue a load of spawnkilling.