I'm generally a roady/triathlete but I have a mountain bike for winter and for off-road events.
There is a fine line between enjoying yourself on a mountain bike and respecting the environment and destroying paths/tracks, etc. Mountain biking is a huge sport and I suspect brings a lot of money into tourist areas. The problem is making sure that there are interesting enough trails for bikers to use - ones that are managed and checked - to make sure that they don't stray into the wrong areas where they can do a lot of damage. Unfortunately, there is a big group of mountan bikers who go out at the weekends and have no idea what they're doing or what damage they may cause to the land and to people. When I go out, I usually follow the routes from magazines, assuming - rightly or wrongly - that they have chosen legal bridalways, etc.
I like to think that I am courteous to other trail users but I get annoyed at the dirty looks I get from walkers, even when I stop to let them pass. The pathways do not belong to any one particular group of users and, if only everyone is sensible, we can all enjoy ourselves without upsetting others.
I take the point above about upsetting dogs and I hope you have a word with these idiots - and I don't want to start a big argument here between dog owners and bikers/runners, especially as we're about to get a dog! - but I have also come across a lot of irresponsible dog owners who don't seem to mind their pets attacking/leaping at/barking at cyclists, walkers and runners. And, no, saying that 'He's only playing, he won't hurt you' does not help, especially when a terrified small child is involved.
Oh, and believe me, mountain bikers certainly do not have the exclusive right to be complete ar**holes - I bike to work from Kingston to Central London 2 or 3 times a week and I am dismayed by the number of cyclists without lights, helmets, common-sense, respect for other road users and the knowledge that red lights apply to everyone.
Wow! I'm really ranting now! Sorry - just had to get that off my chest....