Yes, I love the thought of the free range slippers careering around the park, squeaking with delight as they frolic on the grass. And the dames.... 
OK, dog manners. Firstly, pick up, bag it and don't leave it hanging on a bloody tree.
Put it in the bin or take it home with you.
Be aware of livestock, wildlife, ground nesting birds etc. and ensure dog is under close control while around other animals. Be mindful when choosing where and when to walk. Do not leave gates open (Countryside code stuff, really). Obey seasonal beach restrictions on dogs.
Do not feed other people's dogs. If you have treats, always ask the owner first.
NEVER allow your dog to bounce uncontrollably all over other people, children or other dogs at the park. No, it is not our job to socialise your puppy. And just because an adult is able to walk slowly along, please do not assume they are well enough to cope with being bounced on by your mutt. It is not fair and potentially frightening to children too. Keep dogs off/out of childrens fenced play areas, war memorials and graveyards.
Respect joggers and cyclists - do not allow your dog to hassle them. Though cyclists should also return the favour and USE the Sodding BELL.
Most important of all, if our dogs are walking nicely on the lead and are muzzled, do not assume that your off lead, out of control dog is welcome to come charging over, cannon into us and then simper weakly and say "It's OK, he only wants to play".
One of our dogs is ancient, arthritic, half blind and gets understandably very defensive when strange dogs invade "her" space on walks. And why are bloody Labs always the worst culprits for this?
If a dog is on lead it is usually for a reason, so please respect that.