Yesterday I mentioned (the first half of that tweet is a joke, btw) that I thought many of the world's users of computers are "trained" by Microsoft Windows. I don't claim this is a good or bad thing, but just note that it's currently, as a generalization goes, true. After all, a huge percentage of computer users have been on Windows for 10 to 15 years now.
This morning I was fortunate enough to be part of the team / the audience for a read-out from Human Experience Lab on A/B testing we've performed on the last 2 major versions of the software that I manage day-to-day, and lo and behold: more than one user (almost half of the test group actually) was confused by the use of carets ( ► and ▼) as the visual icon for expandable tree nodes. Several suggested a variant the following: "if you used Windows-like + and – symbols it would have become obvious straight away".
Interesting. My take on the point here is that we have to remain 100% conscious at all time of the audience we are designing for, and ensure that our designs fit them best. And then test it. Improve. Iterate. It works.
So whilst we could technically change that back to + and – in about 6 months, we're actually on the front-end of a coming change already. This means we expect this particular test score to change when we run it again this time next year, even though we change nothing (well, that's not true, we'll likely make the carets more obvious).
Design for your audience (not you), test, improve, iterate. The first part really means you must know your audience first.