Privacy & Security Concerns
I hear a lot of flack about privacy issues using Google Chrome, an open source based browser by Google, which is currently taking more and more market share, but no one ever seems to mind that Opera is not just closed source, but has a proxy function that routes all your traffic through one company's servers.
It seems overwhelmingly obvious to me that Opera holds far more *potential* for mischief than Chrome does.
With that said I am impressed that Opera manage to keep up a codebase that is more or less standards compliant in an age where HTML and it's associated technologies keep getting more and more complicated. You have to admire the Norwegian company for being plucky if nothing else.