While I haven't played Halo 4, I actually agree with the main point of the negative reviews; that Halo has mostly been the same old gameplay with a fresh lick of paint since Halo 2.
I'm not saying it needs iron sights, scripted sequences and all that crap, but I think it's a legitimate complaint to want more from a once revolutionary franchise than retread gameplay (however good it is) with shinier graphics.
Halo 1 was a true innovation, and that exciting spirit of charting new territory, which is a big part of what made it so great, simply hasn't carried over into its sequels, and that's a crying shame.
I enjoyed Halo 2 and Halo Reach, they're quality games, but I yearn for the day that Halo will wow me again instead of just showing me prettier shaders.