Interesting article, and I have also witnessed this sort of thing over the years. At the risk of starting some sort of generational war, I find that younger mtb'ers are often the worst offenders (and I include myself from when I was a teenager and in my early 20's). This could be more of an adolescence thing combined with an adrenaline sport. I've become much more considerate now in my 40's.
Another for the list could be, if you ride on the road to your trails, obey the road laws - don't jump red lights or cut cars up etc. Everyone is watching you.