Do people have issues with the single player, or the multiplayer mainly?
I can see some dissatisfaction with the SP as the COD elements don't add anything and only serve to take away control. Don't think they really fit. The story was okay, but I definitely have to agree with people in the spoiler thread about how it was a bit disjointed and somewhat confusing at times since they glossed over important elements (which I suppose are expanded upon in the novels, etc.), so it was kind of hard to follow as just someone playing through the games. The actual gameplay of the campaign was fairly solid, though I do have to agree with the opinion that it didn't really hit any of the highs of previous Halo games, even if it didn't hit the lows either (e.g. Library, Cortana)
As for the multiplayer - I definitely like it better than Reach. Was not a fan of bloom and invulnerable until shields popped. Ordnance drops are a fun new addition and the progression system is on par, if not better than that of Reach. The maps are adequate, though none really stand out to me as great. My main gripes would be the removal of Invasion (one of my favorite game modes ever since I played it in the Reach beta), though there are those one or two new modes that sort of replace it, and the lack of variety in playlists (do they change at all?) The other thing would be the pruning of the vehicle sandbox. Really would have liked to see some Promethean vehicles (saving them for Halo 5 amirite?)