1. Carefully refined movement settings
I mean really, usually when people think about FPS or even only Halo, they think the most important thing is the weapons. I don’t share that way of thinking. Halo is a very movement based FPS, and with every succesful and non-succesful movement style, I have see a huge impact on how fun I have playing the game.
2. Weapon sandbox
This is the part you’d expect to be first. I put it second because I generally feel that this can have a bit higher margin of error than the movement settings. Movement gets ruined so easily, but weapons can have a tiny amount of imbalance.
3. Maps
Of course the most important thing that comes after core mechanics. Good mechanics always feel even better when you get a map that lets you use then to their fullest. Maps need to vary between complex<->simple and symmetric<->asymmetric. But most importantly, all maps need to give the player a freedom of movement in as good sense as Lockout did. Of course they also need to look as beautiful and unique as a Halo map always should.
4. Vehicles
After those three, there didn’t seem to be much left, but being a huge fan of BTB, I managed to dig this subject out. Vehicles need to feel powerful and be hard to destroy, but not be overpowered. This means that they shouldn’t be able to be destroyed with any other weapons than heavy/explosive weapons. This means no sniping, DMRing or BRing of Warthogs. You should be required to use your mental capabilities to destroy vehicles as they are already too easy to aim at. Of course maps should have enough power weapons and grenades to be able to kill those vehicles. On a side note: Missle Pod was good, it worked well against vehicles, but sucked against infantry, this is why it was so good on BTB maps. If you want better understanding of where I’m comnig from, think the vehicle sandbox of Halo 3.
5. The rest
I figured out I had already handled the whole gameplay there. This part is for everything that is built around the gameplay. Obviously this includes, campaign, Firefight, Forge, teather. But it also happens to include every addition that is made to the gameplay as these things should never be part of the core gameplay, only function around it.
That’s my top 5 with proper explanations behind my views as I always have. I really see no point at writing a list where you don’t justify your points. That kind of posts don’t help anyone.