What I enjoy about plein air painting (in my limited experience) is the spontaneity of the process-capturing the scene in front of you quickly, most times by trying to achieve that through an economy of mark making. This can be a challenge, since I rarely do anything quickly (insert sad trumpet sound).
I consider myself an abstract painter, so another challenge is to take that landscape/cityscape/etc. and try to present it a bit differently.
Getting outdoors to do something you truly enjoy and to do it along with other like-minded folks are two more positives to take away from events like these.