Call of Duty 4 being one of the most amazing video games I've ever played in my life..
does NOT in any way compare to Halo 3.
Halo 3 has more features than Call of Duty 4, such as..
"Theatre" mode. This mode records all games that you have played. At ANY time, you can go back, and watch your games, cut out clips of them and save them, or take screen shots of it. You can learn about what you did wrong for future reference, or just for fun.
"Forge" Mode. You can take any part of an already created map, and move it around..
Like..
You can make certain different game types, and make the maps work especially for them, such as..
Jail break, or cops and robbers.
You can have a portal send someone to a small room, and in forge, block off that room with cement walls as a jail.
You can do tons of other things with forge as well.
UNLIKE Call of Duty 4, you can do campaign mode on cooperative, not only on the same system but through xbox live as well. with 4 people not only 2.
Call of Duty 4 gets very boring just killing people the same way with different guns.
The level up feature is also very good on Call of Duty 4, because of the new features to use.
But, Halo 3 also has a level up feature. but, with no new challenges like call of Duty.
Halo 3 has a lot of different game types, while in call of duty, every single gameplay is based around killing the opponent.
Don't get me wrong COD4 is probably the best game I've ever played, but Halo 3 is above it in EVERY way, in my book.