As a beta-reader, I tend to be rather critical. I can be someone who looks simply at grammar and spelling, to the type of prose you use and whether if it suits the writing and story at hand, to someone who can totally vaporize it to molecules if you wish for that.
However, depending on how you wish for me to edit, that is how I will edit. I will respect your wishes as a writer, and edit it as you wish.
How I beta/edit:
a) I can go through line by line, and nitpick it as such for spelling and grammar
b) I can also tear the entire story apart like an angry Cenobite, and be brutal. I will not hold back to tell you what you have done wrong or done badly, or that you need to improve on such and such.
c) I can edit gently - I am willing to give praise where praise is earned. In other words, I can edit in a 'gentle' fashion, if you rather not I act like an angry Cenobite in my editing.
They're not particularly necessary - I mean, I am a versatile betareader. I will read any style, any genre, and nearly any fandom you could possibly think of. However, please tell me which fandom it is that you're writing, as there are a few that I know, but not particularly familiar with. ^^
In conclusion (my, this is short!):
I'm a friendly person, with a versatile style. I balance my criticism with praise where it is earned, and I'm not brutal for the sake of being brutal. I have been writing for approximately seven years - so editing for as long, so yes. I do know what I am talking about. : 3