I for one played Final Fantasy XI - a good game in my opinion, and I did so for about 3 years. Make of that what you will.
That game also had a "Ninja" class in it... This class could be played offensively or defensively based on equipment and skill-choice. (Had to buy spells etc.)
In short, you either played a DPS Ninja, or a Blink-Tank Ninja. (aka. take as few hits as possible, obviously making him have too little chance to be hit would be a tad OP, which is why I think it's good he has a skill which reduces the damage of the next attack instead.)
I personally don't see the big deal with a "Tank" Ninja coming into LoL. Sure it's not given all the preconceptions to (It seems...) what most of you believe to be a Ninja, but then... isn't that a good thing?
I personally look forward to seeing Shen in action, since in the right hands, he seems he could be a formidable foe (and ally).