Rather disappointing.
First thing I notice - using XP and a black-ish theme, Opera 15 is blue. It's rather ugly and does not conform the UI, neither seems to be changeable through themes. I know XP won't be a priority, but that's what I'm using for now. As for the version number - why did you skip 14? Are you jumping onto the same 'bump the numbers' train Chrome and Fx are on?
Things I don't like:
Very, very crippled customization options - no way to change what's on the only (!) toolbar, no real statusbar
No sidebar, no real bookmarks (like they used to be)
No control over how tabs open - I'd prefer the old Opera approach, 'new tab at rightmost', not 'new tab next to active'
No way to customize gestures - sure, the basic ones seem to be like the default ones in previous versions, but I'm a power user and need customization
Discover is as crippled as on Mobile - no way to define specific feed sources; if feeds were customizable, this could be at least a tiny partial replacement for M2
Empty part of the tab bar acts as a title bar - double-clicking it maximizes/restores the window and does not open a new tab (as expected)
Huge-*ss Google search on Speed Dial with no evident way to change it
No smooth scrolling; you should be able to bring it back with the shiny new over-praised engine, right?