It's the utter and complete lack of complexity of the rotation. To me, it just doesn't "feel" like a real DD spec (because it really isn't). Perhaps I'm too old school and in love with the way that Shadow used to play. I'm not going to sit here and expound upon the complexities of Shadow (because it's not that complicated), or gripe about the 5.0 changes to the spec (because imo they weren't good ones), but there is a bit of nuance to the spec and I feel that nuance is completely gutted when I switch to Holy and DPS. As I said earlier, it's a neat gimmick, but it just feels empty and hollow.