I know Northy disagrees with me on this but I think that - on Expert at least - the changed tank AI in L4D2 makes the tank a much easier proposition than in L4D.
In L4D the tank will incap you and then give you the one or two hits required to kill you. If you are incapped by the tank in L4D you are dead, plain and simple. It's common for a tank encounter to end with only one or two players standing and that's bad news when the inevitable specials come.
In L4D2 the tank will incap you then switch his attention to another player. This gives the team a chance to revive you if the guy with the aggro simply runs and dodges. My experience with tanks in L4D2 is that either everyone is incapped or everyone survives.
Northy will tell you that leaving people incapped to concentrate on the rest of the survivors is the correct tactic in Versus. It is. He will also tell you that a tank wailing on incapped survivors leaves himself open to melee attacks from the rest of the team. He does. But Versus is played on Normal difficulty. It takes several hits to kill a downed survivor on Normal, so the tank is indeed vulnerable. On Expert he can make short work of incapped players and in L4D he frequently does.
By making the tank "smarter" for Normal and Versus the devs have, I still believe, made him significantly less challenging on Expert.