I consider Beta-readers as editors. It is not my job to simply look at a piece of work for simple grammatical errors, before the author throws it up on the website. In fact, authors should personally proof-read their work before passing it on to me, because that is a 'must-do' for every single author who has ever posted.
I've seen written work on this website that is so horrible, it's like they didn't even try to correct their own work, no spacing, no paragraphs, endless grammatical errors and so on. I would like to work with someone who has a basic grasp of punctuation, even if they're not overly familiar with English sentence structures. I will still very happily correct any grammar, punctuation and spelling errors that I come across. They're inevitable.
I will not personally correct inconsistencies and continuity errors, because the choice on how to handle these is up to the author. I will point everything out to you, and let you decide the best way to deal with it. I may also offer suggestions as to how you go about correcting such things.
Corrections to pacing, amendments to plot-holes, fixing characterisation and changes to phrasing will be left to the author. I am not there to rewrite your stories. It would be tremendously unfair to actually do so. I will happily point out what could be changed, and leave the final decision up to the author. If I did these changes myself, it would be own writing, which is more along the lines of co-writing, than beta-reading. It's best to let the author correct it themselves.
If you're going to ask me to beta-read, there are one or two things you should keep in mind.
First, I prefer it when a writer actually asks me to check certain things for second opinions or possible corrections. I will still read the whole thing, but will focus on whatever you feel might need working on, even if it's just to ask me to do a quick grammar check.
Secondly, I would like you to ask me how long it will take to go through a piece of work. Since you have the work, the number of words does affect how long I'll be spending on reading it through before I actually start to point out possible fixes.
Thirdly, Be polite. I am going to spending my free time looking through your work, so it's required that you don't get annoyed if I point out errors. You can be annoyed at yourself for making them. Do take my advice, unless there's a very good reason not to. I won't work with someone who refuses to listen to anything. People can be stubborn when it comes to their own works.
Fourthly, I'd appreciate it if you sent me back a copy of the final corrected version, rather than just posting it. However, this doesn't have to be the case if you believe there are no further errors.
Finally, even if changes are minimal or not required, do give me credit, especially as I have still made the effort to double check things through.