No, its not the XP cap.
No, it’s not the Mark VI unlocks
No, it’s not even the DMR.
The real problem I have with 343 right now is; why can’t I make my Halo 4 emblem my avatar? I feel like I actually managed to make one that looked decent, and if this website were Bungie, I’d have that option. But no, it’s spartan model/crappy presets or bust apparently.