So...issues I'm seeing reported so far:
1. no ability to set minimum font size in Safari 10. (Mine isn't set, so that isn't likely to be an issue for me, but others may have trouble). You can zoom, but that makes EVERYTHING bigger, so in some cases the edge of the page disappears, requiring sideways scrolling. Not an acceptable substitute - what on earth was Apple thinking??
Other than that, it's mostly issues related to downloading Sierra/installing - the same sort of thing you tend to see with any update. So...when I find the time...