By far the biggest reason I've remained a loyal NW customer over the years has been first class upgrades. I used to value other things, however as almost all the FF programs have been devalued, the one outstanding thing about NW is free domestic FC for ANY fare class.
edit...just read my post...I should have said free domestic FC upgrades....LOL. Just goes to show you I much I count on it....it's so rare to not get an upgrade as a PE.....I might fly coach domestic once a year.
Tracy