Over on Mysterious Matters about a month ago, Agatho (?) has published a few great tips on what "Well Written" might mean. And he has them well in order, from the things that almost everyone will agree bother them, to the things that are nice-to-have. I can't say that number 8 (a sense that the author is an interesting person whom I would like to meet) has ever been high on my list for novels, unless you count first-person-viewpoint-characters as "the author".
Since one of Agatho's points was about sentence variety, here's a quick pointer back to Alex Moore's discussion of that subject.
For another definition of well written, Alicia Rasley at Editorrent has just posted a "Quick Turning Points Schema" which summarizes the reversal points in the standard three-act story (such as a screenplay). Call it classic or call it cookbook, but it's something to measure against, and it works.
Editorrent does miss a few beats in the standard screenplay. For example, where she puts the initiating event, there are often two points in a screenplay - the first "Inciting Event", where the person finds out *what* they need to do, and the second "Plot Point One" where they decide *how* they need to do that. The *how* is usually modified at the "Reversal Point" halfway through the story. Examples from Ehow, better fiction and Lost Zombies Film School.