It's true that editors and agents aren't going to jump for joy when you tell them that your friends and family love the story. But does that mean you should exclude them from critiquing your story? Absolutely not.
One of the best critiques I ever got on a story was from a very good friend who didn't read a lot of kids books. Her critique was valuable because she told me where the story was muddled, when actions or words didn't make sense, anytime she had questions or if something was too unbelievable. And that's exactly what you want from a beta reader.
You don't want readers who are just going to tell you your writing is fabulous. If that's all you're looking for, then you aren't ready for a beta read. You also don't want people who are going to change the way you've told your story, insist on alternate story lines or rewrite entire pages or change your character's voice.
When I beta read, I try to point out plot holes and places where the action drags, make sure the character's motivation is clear and note times when the characters might not be acting "in character." I also highlight repeating words or phrases, and let the author know if something in the story doesn't make sense. Almost as importantly, I make note of scenes that work beautifully, dialogue that impresses and things that just plain make me smile.
Before I start reading, I ask the author what they are expecting from me, how detailed they want my critique to be, and when they would like to get my comments back. Some people like notes at the end of each chapter. Some people want overall thoughts. Still others prefer a line-by-line critique. Being up front and clear saves a lot of misunderstanding and hurt feelings.
I LOVE being a beta reader. I mean, how cool is it to get a sneak peek at great stories before the rest of the world! I've also been so thankful for the comments I get back from people who critique my writing. Yeah, it's devastating to hear that those two chapters sucked and I should really delete them completely. But better to hear it from the betas than to send that story out before it's ready.
Like writing, the more you beta read, the better you'll get at it. I was TERRIFIED the first time I did a beta read for someone, worried that my comments would offend, scared that I might miss something important. But keep this in mind: chances are that you are just one of several people reading the story. Hopefully, the author will get comments that are similar to yours as well as some from people who catch other things.
We all bring something different to the pages we read, and that is the beauty of the beta.