The right/down arrow is good now because it is already common, but as you mentioned, it's counterintuitive and time ago there where a lot of problems with that. Like with many new interfaces/mechanics.
+/- is, in my experience and opinion, the best option since it says clearly what is happening and what you can expect. more things, less things.
Up/down arrow in delimited areas is also a good and clear option. For instance, you have some contents in a list or paragraphs and each has a shadow or border that delimits that idea or content and you place the down arrow on the left of the content. when pressed, expands the area to show more and the arrow to point up.
more/less (in words) a few pixels smaller than the normal text is even more clear, but it may affect the design.
Any icon should also have the title with the meaning to help the user, then almost any kind of icon would be ok. They still should make sense, of course. And the text should be descriptive, for instance "more content".