The most notable change to Halo's multiplayer is the addition of loadouts with unlockable weapons and boosts. While I guess it was only a matter of time before Halo joined the COD-ification that has been emulated by nearly every FPS from the last few years, it does change the feel of Halo quite a bit. First and foremost, Pre-Halo 4 multiplayer was predominately about map/weapon control. It was more a descendant of Quake than Call of Duty. While Halo 4 strikes a balance between weapon location pickups and starting loadouts, once you unlock the DMR you are no longer in dire need of finding a new weapon. I found myself sticking to the DMR unless I happened upon something powerful, but it wasn't necessary since I could drop enemy Spartans at range quite effectively. Another change is the highlighting of weapon locations on your HUD, which I actually don't mind since it does make multiplayer a bit more accessible to those players who don't have hundreds of hours memorizing all of the weapon locations on all of the maps.
Returning to the COD-ification of multiplayer, there were a few times early on that I was frustrated by a higher level player putting up a light shield that I could not use, effectively turning the tide of the duel and resulting in their surviving and winning where in previous Halos I would have had the kill. It takes a few hours of play to unlock the base level of all your loadout spots, so in the long term it isn't such a big deal, but it is a source of early frustration.
Lone wolves/FFA mode as it were no longer exists. It has been replaced by Regicide, where the leader had a giant target on them and killing them awards up to 3.5X the points of a kill of anyone else. It's an interesting mode, and it certainly allows new players a chance to be competitive, but it seems counterintuitive to have a game mode where being in the lead for most of a match makes it more likely you will not finish on top. I will admit being frustrated when I had the most kills, best K/D ratio, and most time as King, and still lost the match. It would be fine to have this game mode--it can be quite enjoyable--but it would be nice to also have the option to play a pure FFA deathmatch mode.
I'm also constantly baffled by competitive shooters' inability to learn from Halo 3's ranked multiplayer matchmaking. In my first match as SR1 I was put in a Regicide game with SR43 players. For some reason games have been unable to effectively match people based on skill level and experience ever since Halo 3 did it so well.
Another note is that 343's file share system is still broken and not expected to be working until mid-January.
While my first impressions are a little nitpicky, as a whole it felt good to be back in a new Halo multiplayer experience. The game looks great, controls quite well, though the new grenade physics take some practice and adjustment, and I plan on playing quite a bit more of it.