World opacity is very important for my workflow, and I thought I'd share a word about it for you guys, in case you didn't know. It's been made possible again after a long absence, but still has its bugs (like the leaf thing).
Benefit is that you can now use a color mask to really get all grass (e.g.) off a certain area, especially handy when using wider patches of grass.
I found that using an altitude restraint didn't work as expected; with a final position minimum altitude (e.g.) the iterations come in the right place allright, but when also using world opacity the distribution shader also needs Y to be checked, or they won't show up at all, at least not in RTP.