Must have's in a FPS game(can't do without them)?

Depends on what type of FPS it is.

Deathmatch or team deathmatch style competitive: None of it.

Battlefield style team matches: Level ups are cool but not necessary and sometimes it can even adversely affect game play.

Co-op Style: Gotta have loot drops and a good campaign (basically borderlands). Customization can be cool here but not necessary.

So yeah... Really depends.