The term 'one-trick pony' comes to mind.
Yes, Slender is creepy and atmospheric. Yes, being stalked through a dark forest by the Slender Man is a terror inducing prospect. Yes, it's pretty scary... ...the first time you play. After I played through it a couple of times, I pretty much got the routine down. Go here, watch out for Slendy here, walk there, stay on the paths, go to the building complex first, yadda yadda yadda. Fear comes from uncertainty. If you can fairly accurately predict what's going to happen, then it loses all of its scary factor and just gets repetitive. And Slender is very repetitive. Scattering the pages in different locations, which the game does every time, doesn't add the variety it desperately needs. And while it does get fairly tense after you've gotten most of the pages and Slendy is good and mad, it comes too late to save the weak experience. Maybe The Arrival will be better, but I'm not shelling out however much it costs to buy. In conclusion: it's really not worth playing more than two or three times, no matter what anyone on Youtube says. Of course, if you're taking recommendations from the people that make dozens of Slender videos, you've got bigger problems.