Just to refresh your memory, AA Plat is the same as everyone else's Gold level. EXP is everyone else's Plat. I see their marketing scheme worked on you!

You wondered why DL elites stay with DL. For many of us it makes sense. Out of my home airport, SRQ, DL's lowest published fares are almost always K fares. As a mere Gold Medallion, my upgrade percentage the last two years is very near 100% when originating from SRQ. There are two segments last year I didn't get upgraded: A non-stop SRQ-CVG on a comair CRJ (long story... I usually avoid RJ'sas much as possible), and one segment returning to SRQ from CVG, where I forgot(GASP!) to call at the 72 hour window. I called about 30 minutes late and got shut out. That accounts for about 90% of my flying. The other 10% of my flying is out of TPA, and I am running about 25% or so on upgrades from U fares(yes you CAN occasionally upgrade a "LU-ser" fare, if you play the game right!). So overall my upgrade percentage is running about 92% as a Gold. The ban on L and U upgrades significantly decreases the competition for upgrades, so I don't mind it at all. The 800 mile segments are a big plus, as is the fact that I get 8 of those for every 10,000 base miles. The only thing really lacking in DL's program is SWU's, and since I don't fly any paid international, it's a non-issue for me. I always cash in miles for Biz the few times I go international.