I listen to a great many audiobooks. I recently made the leap to listening to more books than I read, thanks to the many digital resources available - happily! I wouldn’t be able to read 100 books a year any more if I had to sit down to read them.
I've observed a few things from my listenings.
1) Production quality is less important than the performance. I can easily forgive a dull staticky background (sub-standard booth or mastering) in exchange for a voice that makes me feel the story. I was heart broken when one of my favorite authors went (mid-series!) from a native Icelandic narrator with middling production to a "beloved" English narrator backed by swish production values. I’m still holding a grudge about this.
2) There's some very, very bad production out there. I get many of my audiobooks from the library collection, many of which are classics apparently recorded before digital media, in a time where production standards must have been different. I hear page turning. I hear swallowing. I hear near stumbles. I hear people talking in the next room. Any amateur starting on ACX would be destroyed for such poor production, and this is from Random House! Speeding up, slowing down (varying pacing is far worse than just reading faster or slower than you’d prefer). Volume up, volume down. Mic bumps. I swear once I heard the narrator take a sip, and in the same book, read right over a burp (the two might have been related). It's like they locked him in a room and said “You can't leave until it's done! No breaks! Have to pee? Read faster!” This kind of crap always makes me feel better about my early mistakes. Standards are much higher these days:).
3) Using unusual emphasis, once in awhile, keeps the listener alert. There's ten ways to read every sentence using different emphasis. Even a three word sentence. Three ways, with emphasis? Add a question mark, exclamation mark, anger, sadness, resentment, hope, hopelessness, or distance. Considering just emphasis, though, placing it slightly on a word unexpected makes a little hop of cognitive dissonance in the listener, and I find it keeps me very tuned in.
4) It's obvious when the narrator doesn't know what the sentence is saying. When they don't, they aren't able to communicate the meaning to the listener. Maybe you (listener) can get it, if you remember the words of the sentence and reassemble them in your head retrospectively, but maybe the narrator carries on and does it again with another sentence, and then you can't catch up. I think this is the essence of the job of the narrator, and the difference between reading a story and telling it. As the surrogate voice of the author, the narrator is supposed to be communicating meaning with all the nuance available in speech, not just vocalizing words in the order they're on the page. It’s easy to tell (and frustrating) when the narrator doesn’t get the point of a sentence.