Having both lost and regained status on AA and UA, I would offer - in order of value - the following observations, some of which have already been duly noted by gleff:

1. Priority security clearance. IMO by far the biggest benefit if TSA lines are a problem at your usual embarkation points. Without AA Gold status, I would have missed my flights at BOS and SJC. On the other hand, status is worthless at OAK, LAS and DCA where the lines can be long for everybody.

2. Priority Seat Assignment On UA this often means the difference between a tolerable E+ seat or awful E-, particularly as UA may start charging non-elites for E+ seating. Less of a benefit on AA planes which mostly feature MRTC, though I missed the ability to book exit row seats, particularly on the dreaded 757 slave ships.

3. Upgrades. For both AA and UA, this means the ability to upgrade domestic US flights on any fare with 500 mile e-upgrades or "stickers. A huge benefit IMO as most of my leisure travel is booked on deep discount fares. Not so useful if you fly on mostly full Y fares. In addition, status will put you higher on the upgrade waitlist - quite valuable to those with midlevel status (UA 1P and AA Plat).

4. Priority Airport Check-In. As noted above, less useful in the day and age when you can print boarding passes on your home computer or an airport kiosk. But the shorter lines can be a lifesaver if you need to check luggage, change your itinerary, use a paper upgrade, etc.

5. Priority Boarding. Very useful if you never check luggage, even more so if you are not flying C or F. IMO somewhat less valuable as I haven't seen the bins fill as much as they used to, except on SW.

6. Bonus flight miles. Not worth much at the UA 2P and AA Gold levels (25% bonus), much more valuable at the 1P and Plat levels (100% bonus).

7. Dedicated CSR telephone lines. Can be a lifesaver when your flight get cancelled, sometimes useful when attempting to book award travel, but otherwise not a big deal in this day and age where people can go online to book flights, view seatmaps, request upgrades and print boarding cards.

8. Partner promos If memory serves, UA and AA elites have recently received comp elite status in Starwood and Hilton. Also, the discount codes for car rentals can sometimes be useful. This should probably rank higher.

9. Priority waitlist and baggage handling I've never used these features, but they may be important to you.

In general, I would agree with MileKing - The ramifications of walking away from status are nowhere near as significant as many FlyerTalkers think, particularly if you fly infrequently. OTOH, I think it is well worth keeping status if you intend to keep accumulating miles on that airline, particularly if it makes a big difference in terms of comfort (e.g. UA's E+) , time saved (e.g. elite counter and security lines) or upgrades.