I really wanted this game to be great. And it looked that way until I got into it.
Negative:
They removed some of my favorite parts of Halo, (Invasion, Forge World, highly customizable custom games, etc) and instead replaced it with lame content, (Spartan ops). Forge is much less usable because they removed Forge World and now you have very little space and pieces to use. Custom games are less customizable, coupled with the restricted Forge mode nobody plays custom games anymore. Multiplayer is exactly the same as COD, with custom load outs, killstreaks, killcams, and Spartan ops is spec ops with a lame story line and less variation in levels. There are really only a handful of fun/playable maps (excluding the DLC.) I never bought the DLC because I dont want to buy into more cheap product when they should have had many more good maps in the first place. some gametypes you dont do any damage to your teammates so you end up with noobs chucking grenades at you and shooting rockets while their teammates simultaneously rush you so their is so much less strategy and skill involved. you can no longer specify good connection or chatty players or anything that Halo Reach introduced into filtering matchmaking. Playing with multiple people on one xbox is now nearly impossible on some multiplayer maps because the framerate takes a dive, (ex. Compound) previously multiple players was less pretty, but didn't effect play so drastically. the campaign was decent most of the way through but had an incredibly lame and illogical ending regarding the bad guy. not to mention it stuck so closely to the original halo story formula, (escape ship, crash on alien world, piddle around trying to find/save friends, discover terrible threat to whole galaxy, end terrible threat) the side story was the most interesting part, ignoring the main events going on.
Positive:
Great soundtrack. Weapons feel more balanced. The new weapons are fun to use and their differences are more about play style rather than cheap weapons always winning and therefore being used. Maps aren't copy and pasted from the campaign, (Halo Reach really annoyed me in that way). There are other small things that turned out nice, (wielding a pistol carrying the flag, etc)
Summary:
Basically this game would be more fun if 343 had come up with custom load outs and spartan ops etc on their own. the fact that they copied 95% of their gameplay from COD just ruins it. They could have easily gone in a new original direction or stuck with the old halo style and just improved it, either would have been fine with me. This is NOT halo, this is COD with vehicles. I stopped playing it and eventually sold it. Have not missed it since.