What makes the typesetting look professional?
Word processors break text one line at a time: fill a line, move to the next. Books are set by looking at a whole paragraph at once and choosing the set of break points that reads most evenly overall — which sometimes means making one line slightly worse so the three after it are much better.
That is the difference most people can see without being able to name it, and it's on by default. Nothing here needs configuring.
Even word spacing
Because breaks are chosen for the paragraph rather than the line, you get far fewer of the two faults that make self-published interiors look self-published: lines with gaping word gaps, and a loose line directly above a tight one, which is the more visible of the two because the eye compares neighbors.
Hanging punctuation
Look closely at a well-set page and the punctuation at the edges isn't quite aligned with the text — quotation marks and hyphens sit slightly outside the margin.
That looks wrong written down, but it's correct: a comma or a quote mark is mostly empty space, so aligning its box to the margin makes the margin itself look dented. Pushing the mark out until its ink meets the margin is what makes the edge read as straight. Typographers call it optical margin alignment, and it's on for every book.
Widows and orphans
A widow is the last line of a paragraph stranded alone at the top of a page; an orphan is the first line stranded at the bottom. Both are avoided automatically by adjusting where earlier lines break. See Widows and orphans.
Hyphenation
Words are hyphenated at proper syllable breaks using a real US English dictionary, not a guess at the midpoint. Hyphenation is what lets justified text keep even spacing in a narrow measure — turn it off and word gaps have nowhere to go. See Should I leave hyphenation on?
Ligatures
Where two letters collide — an f's hook against an i's dot — the typeface has a single joined shape drawn for the pair, and the formatter uses it. Five centuries of printed books have been set this way. See What are ligatures?
Pages that end level
A well-made spread has both text blocks ending at the same depth. Reaching that costs slightly wider word spacing on a few lines, so it is the one quality decision here that is genuinely a trade — and the one you get a control for. See What is Page fill, and what is a river of white?
Smart punctuation
Straight quotes become curly quotes, and double hyphens become em dashes, matching what a typesetter would do to a typescript. See How do I prepare my .docx?
What you can change
Everything above is quality, not taste, so it's simply on. What's yours to choose — the typeface, size, line spacing, margins, page numbering, running heads, chapter openings, drop caps, scene-break marks, and how pictures are set — is in the formatter's settings. See Can I change the font or page-number placement? and Which interior style should I choose?
Every figure in this article is measured on your own export and printed in the report that downloads with it — see What is in the export report?