Plot's all well, but probably the biggest reason I reject manuscripts is for the voice. And I'm not just talking first person past tense (or third person limited past tense--it seems that the majority of recent novels are written in one of the two). I'm talking getting in the character's head. We are them for 200-400 pages. Their thoughts become our thoughts. We witness events through their eyes. Their first kiss is our first kiss.
See what I did there? I made fun of newscasters, because that's part of my humor. I could have told you "Show don't tell" but I felt that the situation warranted an example, and you never would have known my opinion of newscasters. Does that matter in the overall scheme of things? Probably not. But I'm more likable now, right?
Most of my favorite books won me over by the voice. Yes they had great plot and development and everything else vital to a good read, but the voice managed to keep me engaged and invested. If someone rewrote Moby Dick with an updated 21st century voice, I might actually succeed in reading it all the way through.
Invested... That's what it's all about, right? Get us invested in your character. Make us care whether or not she gets the boy or survives the flood or solves the missing computer mouse mystery. And get us invested early. Unlike plot, voice isn't really something that unveils as the novel progresses. Page one, line one, I want voice.
Favorite authors based on voice alone: Diana Peterfreund, John Green, Jay Asher.