Don't EVER underestimate the enemy team.
Just lost a game to a team with 2 mid laners because we didn't take them seriously. No one warded, we never grouped, and our bot lane kept overextending. Needless to say, the enemy team got pretty far ahead, and their lack of ADC didn't matter, especially considering they had a fed Master Yi, Katarina, and Doctor Mundo, and we had no frontline tanks. We lost a game that should've been easy to dominate because we failed to take them seriously.
So, learn from my mistake as I have: never expect to be handed a win. Play to win until it's over.