I gotta admit that when I started the game I did choose Gladiator because it looked pretty and wasn't prepared for people assuming that I would be the leader, so it usually went to the DPS or a Healer to lead. Because of that I don't think they actually do carry more responsibility, it should rather fall on the person who's done the instance before and sure if everyone's new THEN it can fall on the tank by default since they run in first anyway.
I did stop tanking for a while after feeling the pressure and me being new, this is my first MMO and all that, but I still had to lead sometimes on my bard while leveling, simply because I had done it before. Now I am comfortable leading, but really it falls to the person who's cleared it in my opinion.

(err sorry if it's clunky and hard to understand what I write >.> )