The behaviours in the title of this thread often come up on the forum, I think. I've seen quite a few on humping- particularly dog-dog humping- as well as dogs that play very rough etc.

Here's an article by Jean Donaldson, who sums these behaviours up excellently:
Dogs In Canada Magazine -- » Oh behave: Love and mounting

The article consults Fixed Action Patterns (FAPs) which are innate behaviours that dogs (and all animals) practise, which help in, most noticeably: fight, flight, feeding and reproduction, known as the 'Four Fs' (take a guess at the fourth ). These are triggered by criteria in the environment.

What's interesting is how domestication has allowed for the incompetence of some of these behaviours to arise, despite the behaviours being innate and pre-programmed:
Humping is a behaviour that is notoriously deemed as the dreaded 'dominance'- especially when it is performed on a same-sex individual. However, with looking at FAPs, humping can be seen as a innate, hardwired behaviour that a dog engages in when it is triggered by the environment. A large amount of the time, it is during play- when the dog is over-aroused and engages in this behaviour in order to continue the play session but perhaps also in attempt to control the dog's adrenaline rush. Humping can be triggered by noises, sights, interactions and smells and does not mean that your dog is taking the the conscious decision to 'dominate' another dog or person for some mythical ranking system.

Play fighting also pertains to Fixed Action Patterns. Obviously, fighting for food, 'attacking' other animals and running away from predators are not nearly as essential for domestic dogs as they are for wolves, but they nevertheless still exist behaviourally in the dog's brain, despite to a lesser extent. Rough play, biting and chasing can look quite scary at first, especially when you hear dogs trumpeting guttural snarks and demonised growls. But a lot of the time, the dogs are simply loving the experience of playing and are engaged in these innate behaviours.

However, that is not to say that a dog who engages in Fixed Action Patterns are completely sociable. Dogs that hump strange dogs immediately, or constantly dive into rough play can cause a lot of trouble. These dogs are best seen as 'socially inept' and are most probably unsure of how exactly to act around other dogs, and so engage in these behaviours as they are some kind of 'default' routine, hardwired in the dog's mind. Donaldson talks about the consent rule, which is very useful:
So for those that have dogs which perhaps exhibit these behaviours often and all too readily, it is most likely due to an inability to engage in other socially acceptable behaviours. This is what the dog knows. It is not to do with dominance, power-games or usurping others for ranking positions. Just as a human will suddenly babble when in the company of new people, make stupid jokes with a new girl or bite their finger nails, the behaviours are driven by being unable to engage in something else. Teaching dogs to engage in more acceptable behaviours and rewarding for doing so will help them control their emotions much more efficiently.

Why focus on the "Mounter" alone? I also tend to consider the inability of dogs to signal non-consent to such behaviours a sign of communicative incompetence.

Some breeds appear to me to be signal deficient, perhaps this is setting the dogs up to fail and have harder lives socially, than breeds with a fuller range of expression?

At least with pastoral type dogs not selected for sociability with strangers, you can work harder on socialisation and protect them at key stages, but how would you add signals to a dog that's morphology no longer allows them?