If one wants to be catty: could it be because Barrowman isn't that great an actor and JM gave a lazy performance, whereas David Tennant and John Simm were on top of their game?

Though to be fair: one reason why even that small moment at the end of "Utopia" with "say my name" is insanely hot is because a good deal of the viewers already knows the characters and their backstories. We know the Doctor, we know the Master, and we care because of that. Whereas why should we care about this John Hart person doing a blatant imitation of the Smashed Buffy/Spike scene with Jack? We don't know him. No emotional investment there.