I'd say the Story was decent, not amazing, but not god-awful. The advertising for the game was entirely inaccurate to what the story actually consisted of. And I'd consider myself a huge Halo-lore Nerd.
Game mechanics of course at first are scary because it's changing from what we all know and loved, say Halo 3. Reach being the beginning of the divergence, but people loved Reach. (It's funny, they say they do now, but at the time I remember a lot of people saying otherwise.. It's always the case. "New game comes out- Praise previous game. Hate current one until the next new one comes out.")
In regards to Forge, holy crap it's amazing the things they've done with it, all kudos to the Forge Team for what they've been able to do with it.
This game is not Call of Duty, I have played multiple CoD's. Switching back and forth between say BO3 and Halo 5 they are not the same in any manner. "Hur dur. Aim down sights." So..? You know aiming down a scope before was the same mechanic just visually different.. Right? The only difference is you can now do it on all the guns instead, doesn't mean the AR suddenly becomes a replacement BR.
Perhaps some people just do not like change, or something. For me, 5 wasn't /terrible/, but it wasn't my favorite.
True, people seem to think they don't want change. What is CoD always criticized for? The same engine, same gameplay, same mechanics. So if Halo had the same way of going about new games, how do you think that would go down?
These are just my immediate thoughts on the majority of complaints I see and hear all around the place. So yeah.