As a beta-reader, I have to say that I could be what is considered as either a patron or a reading masochist. I just happen to enjoy helping people out with things that could be said as needing work. I take pride in knowing that I've helped someone; given at least some small bit of helpful advice.Now, this does not mean that I enjoy helping work that someone didn't even attempt to fix before passing off to me. I want you to at least care enough about your writing to make it as best as you can on your own, and then give it to me for a second opinion. If you write fanfiction, I hope that you don't just slop something down that equals a random jumble of thoughts and then are going to have me sort through that.
I also do not want a writer to give me a half-written work and then either suggest or expect me to finish the rest. That is incredibly rude and disrespectful, and since it's not my own work I don't see why I should be either inclined or expected to do as such.
If I give you constructive criticism, that doesn't mean you have to take or even consider it but I don't want you to flame me over it. You want me to beta-read your story; that doesn't mean I'll be nice and say that it's 'Really awesome, I can't find a thing wrong with it!'. Everyone has errors in their work; I routinely read through my own work and hunt for the normal grammatical errors and word choice. When I find errors in yours, then I will surely let you know of them and offer some alternatives or advice.
I. Love. Slash. I adore it; the new takes on familiar characters is something I very much enjoy. I'm not even averse to two characters nobody would ever expect together; I've done it before, as anyone who's read my oneshots can see. Now, I would like the two characters you describe as in 'passionate, transcendent love' to be explained as how they came to be in such a state, if they're either archenemies or haven't spoken more than a sentence to each other. I mean, I enjoy Kenpachi/Gin but they might have been shown together outside of a Captain's meeting once. You need to be damn good to explain that transcendent, passionate love.
In any matter, I will be ecstatic to know that anyone would ever want me to beta-read their stories, but I have certain requirements. They're not outside of normal etiquette between betas and the writers that acquire their services. Please, for my sanity and the safety of my wall.