Avoid excessive use of ethnically altered words and phrases. Showing a character's national origin through conversation can make dialogue weighty and detract from the reader's ability to understand advances in the storyline. Picture a Chinese restaurant in Scotland, where the Chinese immigrant learns English with a Scottish brogue. Imagine faithfully duplicating the exact dialogue of Chinese restaurant employees speaking to Scottish customers. Unintelligible dialogue might work for comic effect in a paragraph or two, but used throughout an entire book might cause the reader to use the book for a doorstop.
Avoid banter that does not contribute to the forward progress of the story or add to the dramatic content of the scene. Unnecessary dialogue detracts from the emotional impact of the scene.
Avoid extreme precision in dialogue. Most English speaking people use contractions, slang, and expletives in their speech.
Avoid excessive use of unexplained technical language. While technical language positions your character as an expert, it leaves the reader confused and without sufficient understanding.
For example:
"When fissile fuel captures a thermal neutron, the subsequent energy and release of fast neutrons provides a means to sustain a beneficial chain-reaction," Professor Schmidlap said, to the kindergarten class.
This diminishes the dramatic content of your scene and unless the reader has a background in nuclear physics, the reader and sometimes the other scene characters think--huh.