Having just read a rant about beta readers, and giving free rein to a number of unsavory things that I just said to the cat about the entire thing... (note, the cat does not care. but he's a good listener.)
I would say a couple things about beta readers and reading in general:
1) always beta read with (or for) someone who shares love for many of the same books you already love. If you get a Sci-fi fan to read your romances or a romance reader to proof your horror novel, you're looking in the wrong place and particularly if they don't like your genre of choice, they're not going to enjoy the story. People who don't enjoy the store at the core of the issue are NOT going to be able to help you fix it. (This will also sometimes keep you from accidentally plagiarizing. A problem I've had is that I love the Liaden books so much I sometimes accidentally grab lines of dialogue almost directly out of their books. My primary beta refuses to allow this.)
2) Don't get a wide variety of opinions: a large book group is essentially worthless. Everyone likes different things. I'm not saying pad your beta group with people who say "oh, that's nice." but getting 20 opinions means you're going to lose your voice if you follow all the advice. I recommend 3. If 2 of the 3 say "change this." then you absolutely should change it. Otherwise, also remember that a beta reader's opinion is just that; an opinion. Feel free to disregard.
3) DON'T argue with your beta reader. For one thing, that'll make them much less likely to want to read anything you write ever, ever again. You're not paying them, so you need to be nice to them. Appreciate them. Send them a doughnut once in a while.
If you drastically disagree with something they said, and you feel some need to justify your decision of not making the change (and really, if you feel this need, sit down and THINK about it for a minute. are you mad because you disagree, or are you mad because you feel hurt? Do NOT go talk to your beta reader if you're feeling all rainbow bunnies squashed about what they said. I know you love your own writing and you're a sensitive artist and all that, but GROW A MUCH THICKER SKIN. You will need that motherfucking armor LATER, so you best get to it now.) then sit down and talk with them. Explain why you like it this way, what you were trying to convey with the section, and see why it doesn't work for them. Trust me, shit that is obvious to YOU ain't always obvious to your readers.
4) Listen to and love your betas and editors. Even if you don't agree with what they said, I've rarely come across either who is saying what they say just to hurt your feelings, or to make themselves feel superior. Betas are perfect, wonderful gems who are HELPING you make your story the absolute best it can be. Never forget that.
5) that being said, if your betas are useless, unresponsive, or prone to just nitpicking about comma placement and not commenting on the story? Then get a new one.
Now, I do quite a bit of beta reading for other authors... a lot of times beta reading comes as a mutually beneficial service that authors provide to each other. For instance, I am Elizabeth L. Brook's primary beta reader and she is often mine. But I also do reading for three other writers... now, all of us write in the same, or similar genres (we're all erotica/erotic romance writers) and so I'm very familiar with the tropes of the genre.
So, I've talked about what to do as a writer, dealing with your beta readers, but let's talk about the other side of things for a while; as a beta reader, what obligations do you have?
1) keep in mind that the author is both giving you the chance to shape her work, and the opportunity to read it before anyone else does. That's your payment. (I often give a shout out to my betas in the dedications of a novel, if I'm actually allowed a dedication. Sometimes I'm not, or sometimes, since I thanked my beta in the last book, she should assume that I still am grateful.) But being a beta reader is WORK. If you don't have time or the inclination to do the work involved PLEASE don't volunteer. As a writer, we often rely desperately on feedback and there's very little that's more painful than an empty email box.
2) Find out what your writer wants out of their beta reading; always ask! One writer might be looking for different sorts of commentary than others.
My personal requirements for beta reading go something like this: don't worry about grammar or editing too much - if you see something really awful, point it out, but otherwise, assume that I have an editor who will nitpick my comma placement after things go a second round. Search for consistency; if I say the hero has blue eyes, make sure he always has blue eyes. Little details like that really bug the shit out of readers. (There is a particularly famous author and in one of his Big Fucking Books, a main female character had blue, brown, or green eyes... this STILL bothers me and I haven't read the book in years.) Plot holes. "Why didn't he just CALL HER?" Story pacing?
When I beta, I frequently do what is sometimes called the "shit sandwich." Which is, basically, don't completely shred the manuscript. if it's that terrible and unreadable, just tell your writer. So what I do is while I point out stuff that doesn't work for me, I also point out stuff that really does; that I find amusing, or cute, or just really awesome. These sorts of comments can keep a writer going.
3) Learn to use your tools; most word processing programs have a feature called "track changes" or "record changes." Please for the love of all that's holy, learn to use it. It makes things easier for everyone.
4) Sum up. At the end of a chapter, or section, or novel, write out a couple of paragraphs, general thoughts. Not specific "I don't think this word means what you think it means" but "I really liked the flow of this chapter, it moved along nicely and I was eager to see what would happen next."
Depending on the sort of novel it is - I've beta read a mystery before - I wrote a section at the base of each chapter summing up what "clues" I thought we'd been given in this Chapter and what stuff I was fixating on (does it matter that the murder happened in January) so that she could see where things needed to be clarified or emphasized, or DE-emphasized.
If you have particular beta advice, either as a reader, or a writer, please feel free to contribute to the discussion!!