As I was busy writing all those sticky threads for the new HW help forums, I also noticed that when I previewed a post containing other tags (in this case, I had [ size] tags), the paragraph break/space just before the tags disappeared. It doesn't do it if you don't preview the post. So, what happens is that the text shows up okay in the preview, but the space is gone from your actual reply, so when you click on "submit," you don't get the same thing that showed up in preview.
Oh, another slightly quirky thing that I just remembered while writing this: the first line that exceeds the size of the quick reply box doesn't seem to activate the scroll bar. I can highlight the text in the box to move up and down, but no scroll bar. As soon as I type one more line of text, so that there are two lines more than fit in the reply box, the scroll bar appears just fine.