According to Wikipedia, A beta reader (also spelled betareader, or shortened to beta) is a person who reads a written work, generally fiction, with what has been described as "a critical eye, with the aim of improving grammar, spelling, characterization, and general style of a story prior to its release to the general public." Believe me, a valuable resource, one not all that easy to find, and I know why. Having been asked by several writers to 'take a look at' their work, I have found it to be a time-consuming project. Don't get me wrong; I love to help other writers. By doing so, I get to read some really awesome stories, and I know how valuable a fresh pair of eyes can be. Sometimes one's own brain just keeps reading what it wants to read, not seeing the little mistakes.
I'm not educated in the english language and my spelling skills are mediocre at best, but after having put enough words into my computer(s) to fill over a dozen books, I've learned a thing or two. I also have a much varied life of experiences to draw on for trivial information. Also, since acquiring an internet connection, I've learned a few tricks about the skill of writing itself. I'm no expert; there are several writers which I follow in one way or another who I consider a far better expert than I am, but I do try to pass on what I've learned.
The most frustrating thing about helping beginning writers is their desire to take my word for it all. After however many pages of corrections, suggestions and comments, it's very disheartening when you get another installment and exactly the same kinds of mistakes are still being made. It makes me wonder if my comments and corrections are being read or if they are simply all being blindly accepted. And I do believe this may well be the biggest reason beta readers are so hard to find.
'Voice' is a very real thing; you might say it is a writer's fingerprint, and we all know that fingerprints are very individual. 'Voice' is likely one thing a reader falls in love with when they read your books; it is something that can draw people to buy anything you write rather than just judging by cover and title. If you allow a beta reader to dictate all of your corrections, you risk your 'voice' becoming muddled if not completely usurped. So, my advice to writers who religiously use beta readers ---> LEARN from what they say. They are experience. They are a fresh pair of eyes, They are an opinion on some matter you may have overlooked. And absolutely none of them will be the slightest bit bothered if you don't make a change they may have recommended. In fact, it is entirely possible, them recommending some change is a way for you to see that they missed the point you were trying to make.
I have asked for a little help from time to time without much real success, not the kind I was looking for, however I have worked with now two professional editors. Both have suggested different spellings for names; one I agreed with rather blindly (in that regard) to my regret, the other I agreed with and then changed my mind. Be willing to learn from anyone around you, but also be willing to disagree in order to preserve your own 'voice', your own story.