Regarding the friendly AI, I seem to remember the old metal-plated Marines in the first Halo being really efficient, losing intelligence each game (even in Halo 2, I loaded the Marines on High Charity up with Carbines and they tore stuff up). It might be nostalgia speaking, though. They seem to want to take cover more and more through the past few games, stacking up on bits of geometry and then not doing anything with it, and immediately pulling off and going to another piece of cover. It makes them seem braindead, because they're not offensively capable in the slightest. Doing ONI today made it very obvious that the Warthog gunner AI needs to be fixed, with better prioritization of targets ("no, not the Grunts, the GHOST, you asshole!"), and full automatic capabilities, rather than the tiny, ineffective bursts they shoot now.
On another topic, I wasn't a big fan of the map pack achievements when they were first announced, but somehow I lucked (the emphasis is important, because these aren't really skill-based achievements) myself into getting all of the difficult ones (the stockpile and double kill from the grave, specifically). That they exist seems to contradict what Bungie said they learned from the Halo 3 achievements: not to encourage the player to play in a specific way. But there's a shotgun-only achievement right there, along with the 'get a kill in rocket race' achievement that encourages players to be dicks and not seek the actual objective of the gametype. It really makes me wonder what they really took away from the Halo 3 achievement outcry.