My style as a beta reader is related to my feelings on fanfic in general. When I read fanfiction, my major expectation for it is that it be as close in quality to professionally published fiction (profic) as possible. Therefore all criticism I give will be with an eye toward raising your stories to professional caliber. Expect me to comment on and correct everything from spelling and grammar to character psychology and motivations to plot structure and scene description if I feel it's warranted. If I feel something is not working, I *may* write an *example* alternative to show you how I feel it might be better done, but I don't expect you to use it; it's just to guide you and give you an idea of what to write on your own.
Be warned: I'm tough, but fair. If you do not deal with criticism well, or if you are only looking for compliments, then I'm probably not the beta for you. I'm a Ph.D. doctoral candidate, and so have seen a lot of scholarly criticism of my writings over the years. I tend to critique stories the way that my advisors critique my writings. Criticism may be extensive and thorough-going. I'm not the person to pat you on the back and tell you that your story is just fine, if I honestly think there are areas that need improvement. However, all criticism will be delivered in a friendly and non-insulting manner and with an eye to making your story be as good as it can possibly be-not toward making you feel bad. I feel that I would not be doing my task as a beta if I didn't try to take as much care with your story as possible. I also will tell you when I think that there is something you have done particularly *well.* I feel very strongly that positive feedback is just as important as negative feedback and I will give praise when it is warranted.
If you are interested in me betaing for you, send me a PM through the site messaging service, explaining a bit about who you are, what your story is about (especially *how long* you think it is going to be) and requesting that I look at your story. There are times when I can't take on more new stories, or when I can't take on things other than one-shots-I do this in my spare time and to strengthen my own writing skills, and oftentimes real life gets in the way. Also, there have been times when someone's contacted me, but as we've corresponded it's become clear that our approaches are too different and we really wouldn't work well together. So-ask first. :)
If you send me your story to critique, make sure that it is finished-that is to say, don't send me a work that you're half-way through writing. The story must have reached its end as you see it. For longer works, I understand that that may not be possible and will critique on a chapter-by-chapter basis; however, each chapter that you send me should be finished.
Despite what I said about tough criticism, at the end of the day you are still the author of your story. It's up to you whether or not you want to use my suggestions, and although I may have extensive comments on your story, I won't try to force you to use them.
I am willing to beta stories outside my fandoms; however, I won't be able to critique canonicity of said stories, obviously. Also, I generally prefer not to beta for authors who are under eighteen years of age.