I would like to preface this post by saying that although this is only my second post on the forums, I have been reading the posts here for a few days and have been a Halo player since the original Halo: CE. Obviously many of the changes in this game are extremely noticeable and have been debated numerous times. However, I believe that there are many smaller changes that have just as much of an adverse effect on the game. Admittedly I did not play much Reach so some of these changes could be carried over from there.
Perhaps the smallest change with the biggest effect on the game is not breaking scope when being shot. This gives a player with a sniper a huge advantage. In previous Halos if you saw a sniper with his sights on you, if you were able to land a shot on him you could escape as the shot would break his scope. Taking away this has made snipers even more powerful than before.
The all around downgrade of vehicles has made all but the Mantis nearly useless on many maps. Vehicles have already been hurt a lot by loadouts and the ability of every player to start with plasma grenades, and making vehicles overheat, have limited boost etc, and blow up from shots from a regular gun has damaged them immensely.
In relation to vehicles is the small bounce that plasma grenades now have. On multiple occasions I've been in a vehicle, had someone throw a plasma grenade at me which lands just short, but then bounces just enough to stick the wheel of my warthog, wing of my ghost, or whatever vehicle I'm in at the time.
Weapons dropped by players disappear from the map far to quickly. One of the staples of Halo is getting and holding onto power weapons. In Halo 4 unless you are within seemingly one sprints distance from a player his weapons will be gone by the time you are able to make it to his dead body.
There is a distinct lack of weapons on every map. Not necessarily power weapons, but weapons that are included in your loadout. In previous Halos these weapons would be dispersed throughout the map, not only to be found and used, but also to replenish your ammo should you get low.
The map voting system gives the players too much choice in my opinion. Normally this would be a good thing, but because players can see exactly what 3 maps they can choose from and there is no random option, the number of maps that are played is way down. Each playlist has one map that everyone always chooses and generally that is one of the better maps on the game, but it decreases the variety of each playlist. The system of either keeping the first map that comes up or vetoing for a random map that was used in previous Halos insured that nearly every map would be played.
The final point that I would like to address is the ability of the flag carrier to use a pistol while carrying the flag. In my opinion this makes the flag carrier far too powerful. Although the flag carrier is still extremely vulnerable from a long distance, if he is able to get close to someone, he will win more often than not. The combination of one of the best close range weapons and the capability of a one hit beatdown makes him far too powerful.
While all of these changes are relatively small and only affect certain aspects of gameplay, the combination of all of them takes a step back in the normal flow of Halo games.