I like writing, and while I enjoy the imagination and creativity in play whlie writing fanfiction, I think there is a need to stay true to the character and the show from which we draw these characters. If not, we may as well be penning original stories.
Therefore, when I have beta'ed in the past, a priority has always been to check for characterization and plausibility. I also do look for grammar and formatting, seeing that poor grammar and poor formatting take a lot away from a truely enjoyable story.
And one more thing, I think the chapters or the stories should read like a piece of actual writing - something that is properly constructed, not just dialogues dashed together with author's own notes as asides.
I may come off as harsh and overly critical in my beta, but I've always made it known to the people for whom I've beta'ed in the past that my edits are purely my opinions. I would love it if you would heed my advice, but I don't write your story - you do. Therefore, you should make the final decision on which of my advice to incorporate.
Having said that, if you decide that my suggestions really do not correspond with what you want to see out of the story, I would appreciate you letting me know and I would not be offended if you move on to another beta. On the same note, if I do notice that you routinely do not incorporate any of my suggestions, I will bring this up as well.
My expectations, as I have said, are fairly simple. Please send only stories that are formatted and you deem to be grammatically and logically sound. Please let state clearly what you want me to help with (if it's beyond general critique of the story) and the timeframe by which you hope to receive my responses.