Storytelling and character development - season long arcs and the way characters grew and changed and were explored. Those are general criticisms - I used Buffy because IMO, it is arguably the best fantasy show made when it comes to those things.

I don't mean to say they needed to do exactly what Buffy did, but that the strengths that Buffy had were those that Grimm lacked and kept it from being as good as it could've been.