There have been enough comparisons to Reach and 4 to not retread on that, but the issue is definitely not trying to look like legacy Halo, it's that things seem almost untextured and flat, plus the world is still and lifeless, even in comparison to 7th and 8th gen games. It has better filtering quality than anything on those for sure, but nothing about it really says 12Tflops of eating monsters for breakfast and all their Velocity engine stuff, especially with the pop in.
The last part may have been due to running on a PC and not actually using the velocity engine, but that begs the question...Why, this late, would a demo not be running on SeX?