DL Silver:

1. Upgrades

2. Check in at first class line

3. Choice seats available for pre-assignment

4. Reduced miles for Crown Club membership (do they really think I'm going to PAY for that?)

What's left? All the other "perks" are being made available, one by one, to the general masses.

1. Upgrades
2. Crown Room Membership
3. Medalion Checkin
4. Medalion Security

I'm seconding FflyinMike:
1. Ability to bend/break most of the rules, most of the time, and not pay change fees, ever. I must have been "gifted" $2,000 last year in saved fees
2. Many unexpected good things happening, too many to get into...
3. Operational upgrades and uncollected certs
4. Exit row advance seating when I don't want to sit up front.
5. Pleather, that one time... http://www.flyertalk.com/forum/wink.gif

1. Being able to upgrade cheap fares
2. More operational upgrades

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In response to OldAztec: Even at merely 6'00", coach isn't all that great.

UA:
1) Elite Check-in and Security (especially now)
2) Economy Plus (oh, the joy of having more space)
3) Upgrades (gotta love those op u/gs!!)
4) *A Lounge Access (although the RCCs suck)
5) Elite Phone lines
6) Elite Boarding (almost always subject to "random" security check if you dash to the gate)
7) Priority tags on luggage (never observed on domestic flights)