As much as I enjoy writing, I enjoy reading just as much (maybe even more). I'm a very picky reader and I have to say that my mood sends me in different directions at different times. When I find a story that I like, I frequently get completely absorbed and can't put it down.
Which leads me to my lamentation. I've found a few stories that have plots I'd really like to sink my teeth into...but when I start reading, I have to put them down. Why? The dialogue is so unrealistic, it drives me batty.
"Hi, Jill," Jack said, smiling at the girl.
"Why Hi, Jack," Jill replied. "How are you?"
"I'm fine, Jill," Jack smiled. "How are you doing, Jill?"
"I'm fine, Jack," Jill giggled.
What is wrong with that? I urge you to say it out loud to yourself. Once you've established that Jack is talking to Jill and vice-versa, it is wholly unnecessary to continue using the characters names. We get it...Jack and Jill are having a conversation between themselves.
Here's something else:
"What did you do with my hammer, Christopher?" Bobby asked.
"I put it on the counter with your other tools," Christopher replied.
"Why did you do that?" Bobby asked in frustration. "You should have placed the hammer back in the tool box where you found it. My Father will get very angry if it is not put back where it belongs!"
What is wrong with this? Again, I urge you to say it out loud. Would anyone actually talk this way (except for our robot overlords who have not learned to use contractions yet)? Stories with dialogue should never have perfect diction and vocabulary unless you're trying to make some kind of point. Something of that nature, I can deal with - but a constant barrage of robotic monotones drives me completely out of the story.
Here's my recommendation:
"Yo, dude, where'd you put my hammer?" Bobby asked his young friend.
"I put it on the counter with your other tools," Chris replied, looking up at his friend quizically.
"Aw, Man," Bobby replied. "Why'd you do that? My dad'll have a cow if you don't put it back in the toolbox where it belongs."
See what I did there? I put slang, contractions, etc. and turned it into a real conversation, something you might hear on the streets. Now, it's okay to have a single character with perfect diction and etc. if you can explain it away as they're an alien or ate a dictionary or something (or maybe they're just a pompous ass) but having everyone speak like that is damaging to the story you're trying to tell.