I definitely agree that overall skiing ettiquette and knowledge of responsibility codes is going Downhill fast, and I definitley blame ski schools for not teaching/reminding people of that.
as far as the skiing over skis in the liftline goes, I used to hate that too, until I skied in Europe, where it's commonplace and pretty much like a cattledrive, you have to force yourself into a spot in line, and YOU WILL get skied over if you don't. I don't purposely do that here, and I try to avoid doing to others as much as possible of course, but I also don't get annoyed anymore if some gaper does that to me.
But the on your right/left thing needs to make a big comeback. I hate how people don't have a clue what to do when you shout it.
As far as the sitting in the middle of the trail thing goes, I definitely hate it when people do it just below a headwall so you can't see them when you're screaming over the cusp, and (as a rider) I HATE it when snowboarders plop down across a trail, when i board, I stick to the visible side areas, and when I ski, I do know that you should stick closer to the sides while stopping, but I also try to NOT take away the powder line edges for other skiers, because that's what I like to ski, rather than the scraped off middle sections. so it's also kind of annoying to have people stopped in the middle of your line.