qSA does a lot more than class and id selectors; those have been trivial since long before qSA came along. You can also do selection on element attributes, parent-child selections (include strict child selection with ">"), and a few other higher-order selector combinations.

Still, I don't mean to slight Sizzle, and certainly won't stop using it. It's just nice to see jQuery compete with Zepto, since apparently there's a demand for a build that small, and jQuery is better-tested and less brittle.