Apart from the ridiculous slowness (in real world usage, not specialized javascript benchmarks), I think the biggest issue is the single-thread design. Every serious problem with the browser (well, apart from the UI) stems from that. The fix has been in the pipeline for what, half a decade at this point? Probably more, even.

Firefox apologists say the silliest things about it ("I don't like process-per-tab because it pollutes the task manager"), but really at this point there are no excuses.