Lobby:
1. Gut feeling. Sometimes there's just something off - I can't necessarily put my finger on it, but it's usually a combination of little details in lobby settings and builds and loadouts that keeps whispering to me that I should leave before it's too late.
2. Other players, the host in particular, choosing horrible (i.e. clueless, not just challenging) loadouts and not willing to equip properply.
3. Lag shenanigans like hearing footsteps in the lobby
Game:
1. Other players not putting in any visible effort. Not worth my time, I'll just calmly quit.
2. Connection problems.
3. Excessive lag that just takes the fun out of the game.