I'm a strong believer in team composition playing a much bigger role than individual skills or performances. In other words, as a support main I really hate playing in a team full of squishy ad assassins. (I'm talking about solo queue games.) What I've observed so many times is that teams with the following three dark traits are doomed to fail:
1. Team members are a massive losing streak. They are likely to be tilted even before a match begins.
2. Picks favor a high-risk, high-reward play style. There are several sub-patterns:
Squishy assassins (think of talon, zed, wukong, shaco, etc.). Those picks are fun to play (getting kills) in early-middle game but rarely win the team.
Weak or unfamiliar champions. Many champions such as Ezreal and Twisted Fate have fairly high skill bars and rely upon specific team compositions and matchups to work well.
Full ad team. The opposite team could just build armor.
Limited crowd control abilities. end up killing no one in late game.
3. Team members are toxic. They flame upon entering the champion lobby.
These three dark traits are somehow related to one another - the more one loses, the more tilted one becomes; and the more tilted one gets, the more likely one becomes impatient to others and wants to try out unusual play style to secure a win.
This dark triad concerns less about rational standards like ranks, and more about the subjective side. Riot has always claimed that their matchmaking system only uses the former category of data, but this doesn't mean the outcomes of the system are perfect. There could be hidden biases and correlations. For example, a lingering question in my head is whether the chance one gets a dark triad team is correlated to their losing streak.