|Grammatically perfect, completely unreadable
Word choice is another little thing that makes characters rise to the top. My New York friend came to the Midwest many years ago, but still refers to little corner stores as bodegas. Nobody in Chicago uses that term unless they were once East Coast pedestrians - we just call them corner stores - so that word reshapes the character. A cruller (a simple, braided pastry) takes different shapes and forms depending on where in the country you live, and can also be called a crulla, an Old Fashioned, or just another kind of doughnut (or donut, if you say it that way). And in Chicago, you better know what makes it different from a Pączki.
I have discussed in previous posts the importance of dialogue feeling genuine (Breaking the Rules With Dialogue), but now I need to emphasize that there are limits. When we pick up on a particular twang of someone's voice, it's always fun to try and capture it in our words; to spell everything out and really understand how it all sounds. Find some dialogue from someone with a real think Boston accent, and write down their words phonetically. Depending on the sentence, it might not even look like English, and yet it's taken from the most American part of the USA.
In a way, that's the problem.
Sometimes, the spoken word is best left spoken. In a recent writing workshop, a fellow writer discussed a story he read that included dialogue from dockworkers in Louisiana in the late 18th century (apologies if I didn't get the reference right). The dialogue was reportedly authentic, but that's not the important part. The most telling part is that, according to the reader, it was unreadable. The language, no matter how accurate for the time and place, was such a distraction that it ruined the story. And when accuracy destroys the actual storytelling, concessions must be made.
Now maybe a Louisiana twist from a couple-hundred years ago is too high a bar to compare, so let's wind it back a bit. A while ago, I wrote a character sketch where the protagonist had a fast, lazy manner of speaking. To make this dialogue consistent, I applied two simple rules: First, the -ing suffix would be replaced with -in' (like the dancin' example above). Second, I would use lazy dialogue words (ain't, gonna, wanna) instead of the proper versions. Two simple rules, applied consistently, and based on the way that voice sounded in my head. Simple and consistent.
The dialogue was such a distraction that my beta readers threw it back in my face (metaphorically, but still).
At some point, we need to find that happy medium. We need to make the dialogue stand out and give the characters their tags, but not be so heavy-handed that our writing needs subtitles. This is best achieved with a second pair of eyes. Get someone - anyone - to read the material, and see if they discuss the dialogue. If they don't, ask. Give the chance for a character to sound distinct, but still be understood.