Just my gut feeling. I liked Halo 1 & 2 just as much as anyone... but I have a feeling about this game.
Sometimes I'm right and sometimes I'm wrong.
When I first saw the concept art I felt deep hype. But then actually seeing the models and how it was being translated into an actual thing... sobered me up.
I anticipate really stiff, jilted (poor) animation, and been-there-done-that gunplay. I think anything outside of the core game (gunplay) is going to feel undercooked and shallow.
This is my gut talking. I will eat crow and admit my shortcomings as gaming prognosticator should things turn out differently, but I don't really expect they will.
Edit: I also think it will sell a bunch whether it's any good or not.
This is a masterstroke in posting. I applaud you