There are lots of little things that I'll miss, including:
F, F, and more F, especially on the 767s to/from LAX. All the legroom in the world, like the coveted "double upgrade", available for upgrades from any fare.
Captain Kramer and all the rest of the employees who kept the airline flying through the years.
The ease of award travel as a Plat is coming in extremely handy for a family vacation to the Carribbean during peak Holiday travel season.
The best livery of any domestic carrier, and IMO, on par with any in the world.
As has been said, 1000 "actual mile" minimums for all Elites.
The ability to get from STL to pretty much any point in the continental US non-stop.
Employees who truly value your business as a frequent flyer.