This seems to have last been discussed a couple of years ago, so here it is again. I'm using Opera 50, and would really, really love to move the Menu button from the left to the right.
The reason is pretty simple: I like the Menu button on the right, like every other browser that I've used in recent years. The current placement makes the Menu button look like just another tab, especially if you have a dozen or more tabs open. I find that I keep clicking the Tab menu button on the right by mistake because it looks and feels like where the usual browser menu would be found.
For me at least this is significant usability problem.