This has just come up on another thread - quite a few posters are confused about the Reply button, thinking (quite reasonably) it is how to reply to the post they're on ie quote it. It can lead to confusing threads as the context of the reply is missing.
This is presumably something that's started to happen since the quote function was introduced, it wasn't an issue before because all you could do is add a post to the thread which is what reply does.
Maybe some renaming of buttons is needed to make it clearer?