I am one of those people who is always finding little mistakes in other people's work, I'm a bit obsessive over the little mistakes in FanFiction stories that catch my eye, spelling, grammar and the like.
I can't say I have much experience in beta-ing, but I like helping other people with their work, I find myself doing it all the time, so I thought it was time to put that into good use.
When I do criticize a person work, I usually do it with gentleness, there's no point trying to improve someone's writing by insulting it.
A couple of things I personally expect from authors I beta for:
x Be patient, as much as I'd love to spend all day, everyday on FF, my life gets in the way, a lot. So I'll be as quick as I possibly can with it.br
x I get things wrong sometimes too, so don't cyber-yell at me for that.