I've always wanted to post this, and I'm on a roll today, so here I go!
1. Location! A quality cache needs a good location, something to see which a person might not bother to stop at or not know about otherwise. It doesn't have to be a great view from a mountian, it can just be a nice section of woods getting there, or a nice park, or something historical or bizzare. make sure the area is not private property, and be carefull hiding caches near RR tracks or federal buildings or bridges - most often caches in these locations will not be approved.
2. Hide! A quality cache should be hidden well, but match it's rating. it should be hidden in a manner where a cacher could find it, but geomuggles probably would not aceessing the area from an angle from which they could spot it. otherwise, it is doomed to be plundered.
3. Variety! A quality cache need not be like every other one in your neighborhood. If the area is full of 1/1's, try hiding a harder one, or vise versa. If the area is full of traditional caches, hide a multi or an unknown (i. e. puzzle) cache.
4. Properly rated! An official unofficial rating system can be found at http://clayjar.com/gcrs/ Live it! Love it! If people seem to be finding the cache easier or harder than you originally anticipated, adjust the ratings slightly on the cache page.
5. Good Coords! GPSrs are not as accurate as many think, And a cacher will most likely be testing your coords once, so it is up to the hider to be confident his/her coords are good. The best way to do this is to revisit the cache site different times on different days before submitting the cache and average any of those which the accuracy is decent. Come back using your averaged coords as if you were a cacher and see how close it takes you. I've had good success averaging about 5 times. If cachers continually note that the coords may be off, check them again and edit your cache page with the better coordinates!
6. Effort! Nothing is more impressive than a cache which shows some effort. Perhaps some historical or other info about the cache location, either on the cache page or in the cache. A container which is camo'd or otherwise cleverly made is an added bonus. Clear and consise directions are a plus. Meeting all or most of the other criteria in this post shows effort!
7. Good Container! Container should be decent and seal well. If it doesn't seal well, expect to have to maintain it more. I like ammo boxes - They seal real well, and there is little doubt whether it is closed tight or not. Obviously, there are places when ammo boxes will not work, so just choose your container thougthfully, especialy if it's in an area where water, ice, animals, or other dangers might be a factor
8. Right size container for the area! Not very often is there a need for a micro in the middle of a million acre forest nor is it safe to hide a .50 cal ammo box in a city alley. With a little forthought, it's not hard to hide a larger container well.
9. Good Contents! You don't need to spend a fortune on contents, but there is no need for broken or dangerous items. Geocaching.com will not approve a cache if they know there are knives, lighters, matches, alcohol, adult material, or other dangerous materials inside. And - NO FOOD OR SCENTED items! Animals can smell soap, food, scented candles, air fresheners, and such and come after your cache. Even an ammo box might not be safe from a crafty racoon or a bear. Plus if any dampness gets in the cache, it can make it really smell nasty. Many people have allergies to the candles and air fresheners, and after being closed up in a cache can make them quite potent when opened.
10. Maintainence! Check your cache on a regular basis. Obviously, caches that are found more often or that are in an area where muggles could find it, need to be checked more often. Containers that are of lesser quality or that do not have a tight seal will need to be checked for dampness or mold. If anyone reports a bad condition or multiple DNFs, check on it! And don't hide one where you can't check on it in a timely manner! Don't expect other cachers to maintain it for you. It's very gracious of a finder to help a cache out, but the responsibility is with the cache hider, not the finder!
11. Disable where needed! If a cache turns up missing or otherwise compromised and you intend to replace or repair it, disable it and post a note as to why it's disabled and when you plan to replace it. You can re-enable it yourself without intervention from an approver. We don't need cachers digging up the countryside if a cache isn't there.
12. Archive when needed! If a cache is missing or you have retreived it, or are moving to a new place and will not be able to maintain a cache, and you have no plans to replace it or let someone adopt it, archive it! Event caches should be archived shortly after the event, when enough time has been given for visitors to log it (If someone is still late, they can still log an archived cache). If you arvhive your cache, you will need and approver to re-enable it if you do replace it, so do not archive if you plan to replace the cache soon.
13. Communication! Check your gc.com emails on a regular basis. Respond to emails concerning your caches. Send emails to finders if you have questions or just want to congratulate them - if your cache is their first, send an email wecoming them to the sport. Be friendly and fair if there is a disagreement. If you are not willing to do this, don't place a cache!
14. Before placing or submitting your cache, read the geocaching.com guidelines for placing a cache! These guidelines can be found at http://www.geocaching.com/about/guidelines.aspx and if you don't read and place your cache according to these guidelines, your cache may not be approved and it will just make more work for the approvers and yourself. Be sure to re-read these guidelines before placing every cache as they change as our national security and other factors change.
That's my input, did I miss anything?