Yes, I can also find it a bit annoying when people change guitar tunings often during their act, particularly when they do it between every number, e.g. tuning from standard to DADGAD and back to standard for the next song. I wonder why they don't organise the set better, so they can stay in the same tuning for a few songs, and then change. Better to have 2 guitars on hand if you use 2 different tunings a lot.

And Anne's point about lots of instruments is a good one - I've seen people do a 30-min support slot and almost use a different instrument for every song. It's as if they're showing off how many instruments they can play. For longer sets, it's find to use different instruments, to provide variety, but in a short set it's better (IMO) to stick to one or two.

(Talking of tuning, the other extreme is the guitarist who doesn't bother to re-tune at all, so I find myself cringing at how out of tune it is after the first few songs.)