Why is this popular?
It is built around the jump-scare, and after 20 minutes of play the scare is completely predictable. Find paper - slender appears, and once one figures out the trick of hiding from Slender (hint:really shoddy AI, with less than stellar sight) the title character of the game becomes the least threatening thing, replaced by the camera-effects as your most deadly enemy. The camera effects... The gimmick of fear-induced shake and blur grows old within the first map.
A new enemy has been added for the PC version as well, though the horror of this unknown is quickly replaced with annoyance at the inconvenience of it's less than hard challenge. It gets one point for somewhat interesting atmosphere, that grows dull quickly.
In the end it seems a sloppy put together, effortless, half-hearted cash-in. This franchise is now established and have a steady base already, but it lacks creativity, effort and passion.
To compare this to Amnesia: TDD is like comparing a 5 AM all-night-on-the-heater McDonald's to a five star restaurant filet mignon; in many ways made from the same stuff, but only one contains the well tuned spices, passion and care that makes it a delicious meal. Man, even that analogy breaks down when you consider a meal is to make you full and a horror game is to make you afraid, and Slender TA fails at that very crucial task where McDonald's don't.
All in all: Utterly worthless. Save your money.