Geocaching.com has a listing of fundamental placement guidelines that include legal and and other restrictions like cache saturation. What are some more informal guidelines to follow when placing a cache?
Here is my list of places to avoid:
School playground - Either on or near a school playground is not good. It greatly restricts the hours it can be found. It's ok to look for when I have my kids but sad to say not the best thing for an older male to be poking around. Nearby public playgrounds is ok but not on or too near the equipment.
Areas of high crime - If I don't feel safe taking my kids there probably not a good idea for a cache.
Homeless areas - Again it's a safety factor. I don't want to be searching around near someone's sleeping area.
Center islands - In my area we have a number of islands in the road ways some with monuments (like old guns). While some of them would probably offer really good hiding spots I don't like searching for a cache with lots of traffic flying by.
There are also some less obvious but still bad locations. Quite recently there was a fair amount of friction among different groups of outdoor lovers in certain area when someone placed several caches directly at established but unofficial wilderness campsites.
It may seem like a good idea, the places are usually somewhat known, some of them has been in use for many decades and accumulated interesting history, but think about how annoying it could be when you are waking up in the morning, getting out of sleeping bags, cooking breakfast, packing, etc. and bunch of people show up and start wandering around, poking here and there, looking for hidden caches.
Most of these locations have their unwritten rules, generally respected by backpackers, however, a lot of cachers do not know these rules and will not follow them, which will cause additional conflicts. I know of at least two such sites that were recently abandoned because someone placed a cache there and because of problems that it caused. One of these sites have been in use for over 50 years.
As you point out, there's a number of legal guidelines they enforce and if the cache contravenes any of these it won't even be published (they check each one before it goes live.)
But in terms of other areas that wouldn't necessarily be advised:
- You point out school playgrounds, but I'd go as far to say around schools, nurseries or other places with children in general. This ends up looking rather suspicious if you're not careful and people are generally vigilant around these areas too. Sure, things will likely be ok in the end but you could land yourself with a lot of hassle if you're not very careful.
- Alleyways. On the face of it, these often seem like good places to hide Geocaches - they're accessible and not too busy as to attract attention. However, those factors don't just attract geocachers, they attract other types too - I've had to remove such caches after reports of used needles being found everywhere. Dog poo also tends to be more prolific in such locations, since there's less of a worry someone will observe and report it.
- Areas where an important event is happening. I was particularly aware of this with the 2012 olympics - as a result, many caches around London were disabled for this period to avoid attracting suspicion and potentially rather bad consequences.
- Areas with constant maintenance where the cache is likely to be often found and thrown away. I found this out the hard way once with a hedge that was trimmed back every month or 2...
Also worth mentioning are caches which are difficult or dangerous to get to - but these don't make them bad caches per-se, on the contrary one of the best caches I've found has fitted this description. But you need to be very careful that when doing this you advertise it as such. A lot of people seek out extreme caches, but someone expecting a nano stuck on a lampost and finding he has to cross the Niagra falls on a tightrope isn't going to be too happy ;)