[Dove]

Mouse gestures.

The "graphics off" key. Admittedly less of an issue on broadband, but still.

Fast page loads.

Just generally spare and coherent design. IE is a behemoth and Firefox is a grab bag. It's rare for Opera to do something I don't like.

And maybe force of habit. I picked it up around Opera 4, when I did a comparison with other browsers available at the time and my priorities, and it emerged the clear and easy victor.