Has anyone tested any of these? I'll show you mine if you show me yours -- A/B tests, that is.

Off the top of my head: blue links beat the stuffing out of "designer friendly" links back on my old site. I'd be astonished if I still had the numbers anywhere, since I don't remember blogging about it and it was over two years ago.

I don't have an A/B test to show you this, but trust two years of look at CrazyEgg: it is absolutely, positively, undeniably true that user engagement is centered above the fold. If your call to action isn't already above the fold, put it there, watch as you get extra free conversions. (Incidentally, I put a couple of them through the copy on my home page, on the theory "Why make people scroll back up if they've read to the bottom?" Every time I add a button, the total conversion rate increases.)