My experience has been that it takes longer to self inflate, and the loft is reduced from original. Not necessarily an issue. The non-thermorest mattresses that I have seem to regain most of their loft through repeated inflations and rerolls over a 30 min period. Just a pain to have to do. It is easy for me to store the thermorests inflated since they are so thin as opposed to the others that are much thicker. When I leave the bigger ones inflated, they do reinflate much quicker to fuller loft when I roll them right before the trip.

You can always blow air into the mattress through the valve to help regain loft, but that has several potential problems for the gear freak. Adding moisture from your breath over quite a few years can make the insides a little funky. I'm sure there are some A.T. through hikers that will bemoan the extra ounce of moisture added to the weight of the mattress from breath. The second is that in extreme cases of reduced loft, the mattress then becomes as much an air mattress as a foam pad, with some of the heat transfer issues. Is this really a problem? The latest and greatest thermorests have a diamond cut pattern in the foam to reduce weight, increasing the air vs foam volume of the mattress. (Isn't it amazing how the more air in a thermorest the greater the price??) Personally, I'm not sure either of these two issues are that important.

I suppose that with really old mattresses, you could develop weak spots at the creases where the edges crinkle.

I really haven't had a problem with any of these potential issues since I don't use them that often, they are relatively new (less than 10 years old), and I do store them inflated as much as possible.