This one fact is probably responsible for 10% of the posts on FlyerTalk! Reduced upgrade success rate has annoyed more customers more thoroughly than any other devaluation the airlines have imposed. I haven't flown the other majors regularly since the years when domestic upgrades were plentiful on Northwest. It was really nice, but those days are long gone.
In late 1998 I was forced to start flying Northwest (I was a United loyalist at the time) and in early 1999 I figured out the game with them. I *really* miss the upgrades that I got with Northwest. That was a beautiful experience. My need to fly to a Northwest hub city decreased in 2001-2002 and I remained loyal to United out of necessity (United having the best service in Denver at that time). However, as United became TED, switching to Southwest was a no brainer. Even though TED is dead, never went back to United as the upgrades were few and far between.
I think the days of frequent upgrades are long behind us unless you are flying a lot. Given that, coach is coach, but no change fees, no bag fees, better routes (at least for my needs, YMMV), makes Southwest the best for me.
I'm pretty sure that Delta is not treating people like the Northwest treated me as a SILVER (and GOLD for one year).