The Great Hall restaurant is a huge missed opportunity. I think space is the issue, it's not called the Great Hall for nothing. But to have a space that could be a by-reservation themed restaurant with a bewitched ceiling, occasional shows (Defense Against the Dark Arts, Sorting, etc), and more than decent food, it would've been amazing.
Although, with what I've heard, as well as then the actual proof I've seen with a survey that was sent to me, Universal was contemplating up-charged additions within USH. They really were considering a Sorting Hat dining experience, but The Great Hall was never actually mentioned by name. Maybe we'll still get something small or maybe that's an addition for a Hogsmeade Phase 2?