I know this can be done manually by creating links on the desktop with different profile directories specified, but that is ridiculous. Opera already has the plumbing so to speak, it just needs a front end to it.
Make a drop down user menu like chrome has to be able to open a window with a different user / profile and create new users / profiles.
This is a critical feature. There are lots of people who have multiple people using a single computer and each user needs to be able to have their own profile without having to resort to manually creating links or having multiple users in windows. Also, there are many people who work with multiple profiles for different specific purposes.
There is no good reason why this hasn't been already added. I'm sure that the Opera programmers could have it added in a day. They wouldn't even have to make the code themselves, they could just lift it from the chromium project.
This missing feature is probably the only thing keeping me from using Opera as my primary browser.