You have to revise your work, probably several times. That means you're looking at structure, pace, and character development along with accuracy, voice, and grammar. Is your dialogue effective? Does your plot build? Do your characters deepen and grow? Does the whole thing even make sense?
One of the problems with revising is that the more you do it, the more you invest in what you see in front of you. The more you revise and polish, the harder it is to recognize what might be a big problem with pacing or logic because you've been looking at it so long that you begin to take it for granted without even realizing it.
That's why a good beta reader is so important. Someone who hasn't watched you grow and nurture your first several drafts isn't as connected to it and can question your ideas more easily. Distance is a great thing.
Not everyone can be a good beta reader. I know several former English teachers who are so used to correcting grammar and spelling that they can't focus on larger issues like plot or character arc. Dialogue using slang can distract them from the characterization. If they see "literature" as something removed from "genre" or popular" fiction (which many of them do), their bias can get in the way, too.
I've been in two writing groups, and neither of them did the job I would have liked for a number of reasons. The first was composed of people who wrote in all genres: poetry, "literary," memoir, nonfiction...and me. I was dismayed to learn that the rules of good writing don't carry over from form to form. Two people in the group wrote well and offered intelligent feedback, but the rest made me wonder why we'd outlawed flogging. I finally left the group when one woman announced, "This is in the style of Gabriel Garcia Marques," and I, with my usual tact, replied, "So why isn't it in Spanish?" Nobody laughed.
The second group was all genre writers. At one time, we had 23 members, but six or seven showed up at most meetings, four of us regularly and the others at random. One always came to complain that she hadn't had time to write and wanted us to commiserate. I was the only crime/mystery writer, and people complained that my characters kept getting into trouble. Fortunately, the organizer ran into family turmoil and the group dissolved before I had to resort to violence...which I would have called "research."
Both groups had problems with anyone pointing out weaknesses, such as illogical plot twists, 40 pages of description in a 50-page excerpt, or characters who changed speech patterns from meeting to meeting.
Ideally, a beta reader is familiar with the form you write, whether it's mystery, romance, science fiction, free verse or financial theory. They have to understand your work and appreciate it, but still not be impressed by it. Yes, it's a paradox, but it's vital. If people love your work, they'll be reluctant to point out problems, which is the whole reason to be your beta reader.
A good beta reader can spot inconsistencies and inaccuracies but still focus on the big stuff. I remember one reader who wondered if my scene might have more impact in a different point of view, and I realized as soon as he said it that he was right. I changed it.
Another good reader stage-managed several plays that I directed, and she understood my rhythms and my visual/aural sense. She grasped that I "heard" things better than I "saw" them on-stage and blocked scenes and beat changes as a form of punctuation.
When I drifted away from theater, I asked her to read one of my novels. We met a month later, and she pulled out the well-thumbed MA (she'd read the whole thing three times, bless her) and flipped to a page with a paper clip on it.
"Do you know there's a huge energy drop in this scene?" She'd even turned down the page where it started, and it showed me that the scene needed drastic cutting. A ten-page scene became five because I had included so much detail that added nothing to the story.
Both those readers have moved away and I don't have emails for either of them. Alas. I have two or three readers now, and they all have strengths, but they all have weaknesses, too. Fortunately, they complement each other. One is great for details and fact checking (you spelled this name with an "ie" here and with a "y" later) but doesn't get structure or pacing. We constantly argue about a turning point coming too soon (I think 90% of the book is fine, but she wants it in the last ten pages, even though I don't write whodunits). Another, who does physical training, has a sense of my pacing and comprehends my rhythms. Her standard comment is along the lines of "I thought this dragged until incident Z in Scene AA." That helps me enormously.
A good beta reader can tell you what bothers him or her without necessarily telling you how to fix it. Sometimes, a casual comment like "this seems to start more slowly than I expected" is all you need. Or, "was that supposed to be funny?"
A good beta reader is worth his or her weight in chocolate, so if you find one, cherish him or her. And DON'T give him stuff that isn't ready for another pair of eyes. I don't like to show anything until the fourth or fifth draft because by then I've fixed most of the typos and can mention specific concerns, such as shifting POV or a strained plot point.
Whatever your beta reader tells you, listen to it. You don't have to change everything but remember that this is a preview of how other people will respond to your book. Think of it as a first date that you want to go well. Otherwise, what's the point?