I don't believe you have to necessarily be a rah-rah, fiery sideline speech guy to be a leader. There are guys who don't necessarily say a lot, but they lead by example - by working hard in practice, and doing their job during the game. By all accounts, Mitch Leidner was the de facto leader of the team the last couple years, and I don't think he was a yell and scream guy.
I do get the feeling that Celestin seems to be a leader on defense, and I suspect Richardson, too. On the offense, I would guess Rodney Smith is respected by his teammates.
But in the end, I have always felt that the leader is the guy who goes out there and makes a play to help win the games. speeches are all well and good - but a key sack, or a 20-yard run speak just as loudly.