I'm new to opera - been on firefox mostly. Got a new computer and so decided to give opera a try (on ubuntu).
Mostly like it - one small thing I really don't find user-friendly however is the tiny, tiny range at the edge of the browser that allows you to resize the window when not in full screen. I feel like I'm doing brain surgery every time - the range is so small that a few pixels off and you miss. And forget about resizing in both direction (using the corner, which seems to have a 2x2 pixels).
I would strongly recommand either making that a setting you can set (e.g. how wide the sensibility is) or then make a useful default. Probably taking into account that on modern screen, a size in XX pixels may not be as wide in absolute value as one might think. I suspect this may partly be my problem (computer with greater pixel density == tiny range in mm on the screen).
I'm a young fit guy, but older people with perhaps less precise dexterity/vision would absolutely find it impossible to do.