jQuery popularized use of CSS selectors in clients, and probably had a role in browser vendors adding querySelectorAll.
implicit iteration (obviating for-loops) across a node-set, type-agnostic starting points (constructor accepts a snippet of HTML, a selector, a node or another jQuery object), and simple chaining (functions return 'this') enable concise code
on top of this it smooths out DOM API inconsistencies and quirks between browsers