I must have missed a few lines in 3 or Reach but I had to google Didact for I don't remember any mention of him. They all seemed to know exactly who he was right away as well, maybe they could have explained it a little better instead of just throwing him in there. Also I agree with OP, the change in sounds for the warthog was hands down one of the worst mistakes they made, this dying chainsaw in a vat of dicks noise it makes is ******** my ears off.
I also hate how they felt the need to remove vehicles that were in the past games and not replace them with anything. Less is NOT better 343.