as silly as it sounds, for me (less than a year in), it's the other players!
I find I have the most fun when i'm taking off of a runway with other plays for a common goal... The freedom to do what I want lonewolf style is always nice for those sorties a guy's not sure how much time he has.
I realize that the "horde" is an unfavorable term at times, but I just seem to have the most fun on those sorties with other players attempting to achieve the same goal.... Even if that goal isn't a "base capture", it's still fun. I suppose that's why I enjoy the Special Events and scenarios; I feel like i'm ACTUALLY part of a team instead of another Knit.

It would sure be nice to tweek the Pickup Missions to attract more pilots somehow, but simply using it more would also help. I was thinking the other day of posting missions overlapping one another even if I wasn't in them...
.... At any rate, My personal agenda includes saving/helping at least one friendly per night, wing up on someone and watch their 6 until i'm out of fuel or ammo...
Secondly it's to get one kill from my b-17's gunners... Gunner's need action too!
