I enjoyed halo 4 almost as much as halo 3, I figure there's no where better to go for a good answer than here.
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The campaign, while enjoyable the first time through and fairly well written, gave me no desire to replay any of it. In contrast, I could play at least some of any other halo campaign for hours on end. Forge had a few new features, but key parts of reach's forge such as precision editing, interesting places to forge, and a working save system makes it an overall downgrade. The custom games options were a shadow of what was in reach and the file share system was broken for months. Spartan ops was okay, but firefight was better. The multiplayer took what people hated about reach and added more of it. The new enemies were touted as something new that required strategy to take down, but in the end were nothing but bullet sponges. The new weapons were unnecessary and just served to emphasize the uselessness of the covenant weapons. Loadouts felt very un-halo and ruined any aspects of arena shootering left in halo. The music just didn't sound right. The only good maps were remakes/spiritual successors. The armor looked stupid.