If you use really simple class and id selectors, the qSA-based selector engine may be fine. However, I think a lot of devs would be surprised about how much Sizzle is doing for them.
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.
qSA's design shortcomings have been well documented, John Resig did a blog long ago about them [1].
The list of what you lose if you remove Sizzle is in the native selector file [2].
[1] http://ejohn.org/blog/thoughts-on-queryselectorall/ [2] https://github.com/jquery/jquery/blob/master/src/selector-na...