I think there are a lot of qualities to really good sentence-level writing--the actual writing part of writing--and a lot of them aren't intuitively obvious, but most of them can probably be learned and developed, unless the would-be writer is tone-deaf.
And now I wonder if I can break the basic elements down, as I see them.
First of all, there's muscularity of the prose, which is comprised of a bunch of interrelated things: lack of scaffolding (by which I mean unnecessary prepositional clauses, qualifiers, weasel words, and all those things that indicate to the trained eye that a writer isn't confident in his ability to get his point across without shoring it up), excellence of word choice (concrete nouns and vivid verbs, adjectives and adverbs chosen to be evocative), narrative urgency. There's not leaving descriptive holes, and there's not overloading the narrative with description. Tautness, or lushness, or redolence--all of these things can make for powerful prose. Knowing what the words mean. Knowing how to choose and use the right one.
Second, there's rhetoric and rhetorical technique. Parallelism, meter, imagery, repetition, variation, grammar, inventiveness, paragraph and sentence hooks, "line of direction," logical linkages and logical disconnects. It may not seem evident that meter is important in prose as it is in poetry, but it is... it's just more subtle.
And then there's that old bastard, telling detail and grounding. Kinetics. Putting the reader into the setting, into the character, into the body of the story. There's any number of techniques for doing this: sensory input, making sure that the chair the character sits in isn't just a chair, but Zelazny's "orange chair canted at just the angle at which I liked to lounge" (quote from memory), evoking the characters subliminal awareness of his surroundings--and the coarser but still useful tools of internalization and outright description.
It occurs to me as I write this that I find my definitions of sentence-level techniques spilling over into every other aspect of writing: narrative, plot, characterization. I seem to have hit a place in my writing where it's hard for me to consider one aspect as discrete from anything else. As Velvet Brown said, "They're all jumbled up together. You can't cut one clean."
Not sure this will be useful to anybody, but there it is. What do you guys see as the hallmarks of good sentence level work?