In reply to Cathy:
> So what do you consider to be correct etiquette for this? (I know what I think, I'm curious to see if we agree.)
> what to do if you find krabs/quickdraws left on bolts or anchors?
I would have a discussion about trying to work out the motives of whoever may have left the gear in place. If it's a single krab half-way up a pitch it's most likely to be a bail-krab and the route most likely would be left in a better state without it getting in the way. Conversely, a screwgate on an anchor would suggest somebody thought there was reason to doubt the safety of the anchor without it, so might better be used and left in place, if unsure at all. Obviously, a full set of quickdraws in place must be left there, but a single one on a bolt in the middle of a slabby pitch would be a grey area, as it could be a bail-draw or conceivably could have been left deliberately to help a future ascent if, for example, the bolt would need a long reach to clip otherwise.
> etiquette concerning leaving your own ropes and/or quickdraws in place, or using those of others found to have been left in place?
This is quite crag-dependent. On some harder sport crags, leaving clips in for hours, days or weeks is quite common and it is accepted that others will use these for their own ascent. On easier sport crags it's more likely to suggest that they've just been left there for a short while and that the owner is still around, so worth checking with those climbing nearby.
From the other perspective, it's certainly best to pull ropes quickly after descending to let others climb after you if they wish. And if I were to leave clips in a route for another attempt later, I would have no objection to others using them, but I would consider it polite, assuming I was very close by, to be asked if that's ok first.
> And what exactly do you have in mind here?
> different techniques or styles of belaying that aren't common in the UK, and whether or not they're a genuine danger such that you need to intervene (i.e. very rarely)
Things such as Münter Hitch belaying or even waist belaying for top-roping, no-hands grigri use when a climber is hanging, as well as leaving plenty of slack out when the leader is high up on the pitch with a clean fall zone. Many such examples could be either fine or not depending on the circumstances, so unless you have a huge amount of experience to know better, leaving it to the people involved to make those calls would certainly be best.