It is the tank meta, but the TRUE tank champions don't fit in, simply because they are outclassed.
Shen and Mundo can be super hard to kill, but they can be ignored somewhat.
Mundo has a slow. Nautilus has an AOE slow, A root, A hook, An AOE knockup
Shen has an AOE taunt and cant really put out damage. Amumu has an AOE root and a stun. Amumu also can put out some good damage.
Another true tank champion is Sion. He is capable of putting out some damage, but it can be avoided. He also has two AOE knockups.
To me, an effective tank needs to be one of two things (or both)
-Tanky and can put out damage
-Tanky with good CC
A champion that is just very hard to kill doesn't really come in handy(They are the best for tanking a tower though, but others can do it almost as good). Assassins can jump right on your carry or whoever, Mundo is just going to put on some damage and be super tanky, but nobody is going to focus the Mundo anyways. Nautilus could knockup that assassin, root him and get him killed super easily.
Shen is really cool, but I think he needs a little more CC or something. Maybe if he lands his Q, his target will be stunned if he autos them 3 times while it is marked. Something like that. Could make his build a little more interesting.