1. Medallion Upgrades - DL still has the best chance of getting a free upgrade on domestic flts. (Yes, CO and some others have Elite upgrades but they are constrained next to DL). DL's FC product may be 'limited' in some views but I'm pretty sure that its still better than most domestic Y products. When the company is making you fly as low cost as you can DL offers an optimal ride. I fly on a lot of $200 or so tickets and I've been in F for the majority of it.

I honestly can't think of a #2 answer though - upgrades are the last 'line' holding DL in much esteem in my view. The value of the mileage awards and various secondary benefits has been in such decline I don't think they offer any sort of competitive advantage over most other airlines Elite programs now.