I'd say consistency is the biggest factor, especially in a tournament where one bad game can end a team's championship hopes. A strong opening partnership helps, but teams also need bowlers who can handle the pressure at the death and finishers who don't panic during a difficult chase. I was looking back at the Big Bash League winners list at https://cricketin.org/blog/big-bash-league-winners and it's interesting to compare the different teams that have managed to win the title. Some seasons come down to individual brilliance, while others seem to reward teams that perform well across the entire campaign.