Today, while playing Black Ops, I discussed some elements of gameplay that I enjoy in Halo more than Call of Duty. Having nothing better to do at the moment, I shall list them:
1. Spawning: I don't know much about the spawning system in Halo, but it definitely feels/plays better than the Call of Duty spawn system.
2. No-Scoping: In Call of Duty, no-scoping is mostly luck. You simply point your gun at the enemy without aiming down your sights and hope that you get lucky. In Halo, the bullet will always go exactly straight, so that no-scoping in Halo is all skill/timing. I do know that no-scoping in itself isn't realistic, but when it comes down to actual GAMEPLAY I believe Halo is better for no-scoping.
3.. Vehicle Gameplay: Before I start off this mini-paragraph, allow me to say that my favorite Call of Duty is Call of Duty 3, where vehicular gameplay was allowed. Simply, I enjoy being able to escape my normal viewpoint and get in a car or motorcycle and start running people over or launching large balls of flaming plasma. In Call of Duty, the closest thing to vehicular combat is using certain killstreaks that allow you to use vehicles. However, many of these killstreaks are aircraft, and you can not have multiple aircraft up at one time in Call of Duty, so there are no vehicle V. vehicle fights as in Halo. Also, in Forge Mode, I can set up maps to have many vehicles for epic chopper/falcon/warthog/etc battles. In addition to those factors, you are also very vulnerable and do not move while using killstreaks. Any opposing player can come and get an easy headshot on you with on resistance. In Halo, you move with the vehicle and are not a sitting duck. You may also use the vehicles to quickly transport yourself across the map, whereas the fastest way to get across the map in Black Ops is to sprint, and you can not shoot while sprinting, making yourself an easy target.
4. Customization: It seems that in Black Ops, you can barely change your appearance. Also, the perks you choose affect your appearance, so for example even if I prefer the way I look with Lightweight, I'll go with a different uglier-looking perk that is better for gameplay. My point is that the appearance customization in Black Ops is limited, and even with what you have you do not have much control. In Halo, I can choose to change my armor to many various pieces that are unique, and change the color of my armor to have its own special look. In Halo, when I play free-for-all, I can often recognize players by their armor (Oh hey, it's the guy with the solid white armor again), whereas in Black Ops I have very little to give me clues as to who I am seeing if I am at such a distance as to where I can not read their name (<- run on sentence).
5. Forge: Simply put, Halo lets you alter maps and put in or take out many things as you please, whereas you must always play with the same maps in Black Ops. (I considered adding this to the end of #4: Customization, although I decided not to because section #4 was more about customization of appearance while section #5 is about gameplay customization.
6. Playing With Style: This section is more about how I feel personally, without any numbers or concrete facts to back it up. It seems to me that when playing Halo, I can choose to play regularly or with a sort of "finesse". Because kills take so long, you are allowed to play around a bit more, and maybe jump around or do something funny. Because the player sees what's happening around his corpse instead of a killcam, you are allowed to do whatever you want to the body for a few seconds as a way to insult the player you just killed or you can simply leave the body as it is. Black Ops has extremely fast kills, in which there isn't time for much except "I see the enemy target, I'm aiming at him, I'm shooting, I won/lost". While I'm not saying that there is no room for style in Call of Duty, I believe that there is MORE room for style in Halo.
6. Medals and Tracking Stats: There is a medal for practically anything you can do, and you can go online and look at how many of each medal you have gotten. I could be mistaken here, but I do not think that you can track your feats as easily in Black Ops (I don't I can check how many "One Shot, One Kills" I've gotten in my life, and I certainly think that I can't check that stat online).
I would put more reasons, but I have to go to bed. This post was a kind of rant, but I still hope that someone says something good to this post other than "reachin' to the choir dude" and actually responds with good info.