Opera is a handy alternative browser to have around, but its installation routine has one major annoyance: it automatically sets itself as your default browser without asking if you mind. Opera says it is working on a solution, but hasn't set a time frame.
While it's easy to miss the "set as default" option for Chrome, Firefox or IE, that choice is an always-visible part of the installation routine (though that hasn't always been the case). However, for Opera, currently that choice doesn't show up as part of a default install. Despite not asking for permission up front, if you switch your default browser to another program, Opera also pops a dialog asking you if you want to switch back (though it does give you the option of disabling that question in future).
I contacted Opera to ask why this design approach had been taken and got this explanation from VP of desktop products Jan Standal:
Since Opera 11, the option for choosing not to set Opera as the default browser was moved to the 'options' part of the installer on Windows. This, along with graphical/layout changes, was done in order to make the installer faster (reducing installation time from 2+ minutes to less than 10 seconds) and as easy as possible to use. An unintended side effect of this was that some users have reported setting Opera as the default browser by accident. We are listening to this feedback and will investigate possible alternative solutions.
Here's hoping that change comes through soon.