It took me a few times to realize that I could do bombs or slingshots by clicking in empty space. The tutorial makes it look like you have to click on a bubble. Maybe the black circle in the tutorial should be removed?
I also got a little confused in the beginning about par vs lives. At first it seemed like par was the only important thing, since that's what pops up after the level. The lives icon at the bottom wasn't particularly noticeable to me. I just kind of figured there were only N levels, until I finally realized that dropping bubbles was making me lose lives.
Edit: Oh, and you can multi-touch and set off multiple slingshots at once! Secrets secrets...