Data Structures really depends on the university. At my school, it’s a bit of a weeder class too but I didn’t find it that hard when I took it my freshman year. It’s just a lot of work and a lot of material. I can see how it would be difficult if you weren’t familiar enough with the programming language used (I had been programming in java for 4 years already). I’m a student grader for the CS department at my university and have graded both Data Structures and Intro to Computing. The most common mistakes in data structures seem to be related to syntax, recursion, and some of the more complex tasks related to some structures (such as removing certain elements from linked lists and binary trees).