I would like to chime in here as a "part time" DL customer. Previous CP with US, and current PLAT/CO* with CO.
I LEFT US due to the botched merger. No more "wait until next quarter" for me! I have found a home at CO. In the meantime, I tried NW/DL/CO/UA/AA, and found that I would be happiest with CO.
For me, most of my flying is NYC-ATL/ORD/Florida/east coast, with a few LAX/SFO's thrown in. The BIGGEST thing that turned me off about DL-NO SEGMENTS! DL needs to realize that we all dont fly ATL-Europe! BUT...my ticket spend flying east coast is probably 3-10 TIMES what others flying transatlantic is! I average about $900 one way for FULL Y fares. DL is not very accomodating with this, due to no EQP's/EQS's.
I do remain premier with UA and will be 1K. I agree that if you have a 4th level-aka 150K/150segments, make it KNOWN, with DEFINED benefits. As a CO*, I get VERY little. Why not make it a KNOWN level, and TELL the AGENTS to take care of the customers.
I also do not like the fact that ATL-EWR does not get meals. Why cant DL provide a competitive product to CO on this route. When rushing from all day meetings to make a 530pm flight, the last thing I have time for is a meal, yet CO seems to feed me on the plane every time!
As far as upgrades, I feel a "pay" instrument is fine. Just make it reasonable. Less then the current $50/500 miler from UA. More along the lines of $30/500 (ala AA) and give out 500's with 10K flown OR 10 EQP's!