Today's standard feature that Opera is missing is support for relative paths on the command-line.
Given files and their locations:
Those will fail in Opera. Passing relative paths to Firefox and Safari works fine. This does fail in Chrome though, but there's a bug for it. It also fails in IE.
This should just work (for any program that accepts local paths) and it's a shame that Opera never implemented this. It's crazy that even example 3 doesn't work. I was going to file on this years ago, but someone beat me to it way before that with bug DSK-138956. Hope this one gets fixed and hope it gets fixed in Chrome too.