I'm talking multiplayer, Reach played very slow compared to the other Halo games due to slow movement speed and people being forced to time their shots, I don't like gameplay to be as fast as COD and imo Halo 3 had the perfect pacing, Halo 4 is closer to that.
The AAs were too powerful in Reach, jetpack lasted too long, camo slowed gameplay down even further and Armour Lock was just a massive crutch, it was really frustrating to put someone down to one shot then they become invincible, again this slowed the gameplay down even more because you had to wait longer to finish the kill. In Halo 4 the armour abilities are a lot less powerful and they take more skill to use. I also have to add that in Reach players were using sprint as a get out of jail free card so to speak, they would sprint into the middle of a battle and when they began taking shots they would run back behind cover just because they can, this does not encourage skillful gameplay. In halo 4 this problem has been fixed somewhat because your sprint is lowed down when you are being shot.
There is no bloom or spread in Halo 4 which were both big problems in Reach and Halo 3 respectively because it made one on one battles more random, there were a lot of people getting lucky in Reach from just spamming shots and winning the battles.
Reach had hardly any good or memorable maps and the majority were extremely mediocre, granted Halo 4 has the same problem with 4v4 maps but it has some great big team maps. All the other Halo games have been known for great and now classic maps yet none of the maps from Reach will gain that kind of recognition.
So those are the main problems I have with Reach, I'm sure I could think of plenty more but those are the ones that come to mind whenever somebody asks me. This is all opinion of course and I'm sure you will disagree but I find you saying that there is more a difference between the COD games than there is an Halo a bit absurd and hard to understand, because it seems as if COD make no significant changes whereas Halo clearly has done because they have a safer fanbase, I see no problem with that but I personally don't enjoy the games.
The armor lock was stupid, I agree. Camo I liked and hated, so on the fence with that, but I liked the sprinting. Perhaps that is because I got good at using it, and perhaps because thinking of Spartans getting tired running so quickly (like in Halo 4 - feels even faster) is dumb IMO. If you thought Reach played slow, I'd think you'd like sprint. So far Halo 4 grenade damage feels really underpowered to me. There are new guns which is fine, but I wish with the introduction of this "AI" race/dicact that they were more diverse and not a near carbon copy of the already established covenant and UNSC guns. For maps - Boardwalk and Countdown were cool.
I think CoD changes more due to how Treyarch and Infinity do try to 1 up each other, and I think they both release a more different feeling product than anything the 360 has seen for Halo, which all control nearly identical. Halo plays
the same; the maps and items to use is a different issue entirely from how the game plays.