To me, 6s is much more dependent on teamwork, or at least at the levels I've played at. Everyone in my team knows that they have a role to play in the team, and that if they do not adhere to their role adequately, the team will face the consequences. In 6s, I feel that I know each member of my team much more than I have for any HL teammate I have had, and that group dynamic is what teamwork strives on.
Off-classing, as annoying as it can become in 6s, is essential as it adds that bit of variety. It maintains an element of paranoia that keeps your team thinking on their feet, whereas in HL the paranoia comes from weapon choices, and there are a fair share of unlocks just feel gimmicky.
Also, the obsessive compulsive part of me doesn't like asymmetrical maps.