I've experimented with becoming a beta reader. But, because my initial editing experience is all from within the scientific academic communities, it doesn't mesh well with fiction. I'd love to have the opportunity to develop the skill, but I absolutely understand writers who don't want someone to be learning on their work, and thus being decidedly unhelpful. I hang out in the beta please community (I'd have to check what it's actual name is) but there hasn't been a request that I can fill at a time when I have spoons, so, eh.

And there is a really good professional writer piece about that horrid critique session approach, which I remember being enlightening reading. I'll see if I can remember who it was by, and link it.