I played through Truth and Reconciliation on co-op Legendary over the weekend (yeah, I'm only that far into it. SKYRIM).
Couple quick thoughts.
First, the gameplay has aged very little. The AI is still pretty sharp and often surprising, the level design is still fantastic, there are still too many waves of dudes dropped off from the gravity lift, etc. The new weapon effects are largely fantastic, and do add a great deal to the feedback loop. I largely enjoyed the visual update, gonig back to the classic mode was often jaw-dropping, and I think the tweaked cinematics are largely for the better.
Against this, some small but important problems with the new visuals. One of Bungie's underlying combat principles is one of clarity. I've ranted at length about this before, but it's a principle that dictates key information is well telegraphed to the player. Things like grass and bushes so tall that we can't see straight (I spawned in a shrub on the cliffside once, couldn't see where I was, and walked right off the cliff) and insufficent feedback when Elites shields popped indicated Saber didn't quite grasp that.
They are also averse to darkness, which is unfortunate. The outdoor section was so bright, it almost didn't even feel like a night time level. I had this same observation in the second level ("Halo"), during the underground section. I was doing some exploring around the edges of that area (lookin' for skulls), and the place was well lit and wonderfully detailed. I toggled to classic mode and found I was in total darkness - and had to turn on my flashlight to see. The darkness was almost oppressive, with mysterious structures emerging from it, disappearing into the depths and distance. There are some places where I preferred the old art to the new, but that's going to be the case with anything but the most slavish updates. Overall, the new visuals are pretty great, but much of the mood has been lost along the way. (Similar to what was lost with the lack of sun-behind-the-tree effect in the second level.)
But you know what? I played Halo on Legendary co-op online with a friend. And it was very, very fun. The fact that I could even do that is enough to make me pretty happy about the package. The new visuals are mostly quite welcome, and the game is as fun as it ever was.
Please 343 - make Halo 4's base player traits hew much more closely to Halo 1/2/3 than Reach. I'll post more about this later on, but that was one of my big take aways from the romp. If you took Reach's player weight, momentum and jump and applied it to Halo 1, the quality of the experience would drop straight through the floor. It was so wonderful to be agile again. Not just because it's fun to be, but because the game design is better for it.