I make up my own achievements.

1. Craft and collect the Commando Custom-set from each campaign.
2. Collect each Commando elite vanity.
3. Ongoing: reach the next 5k kill mark.
4. Ongoing: be creative. (e.g.: create new achievements!)

This way, it's less about just grinding for xp or getting gear to sell. Oh, but I would like to see more quests.