Workflow: Opacity lets me define factories to quickly generate images as I iterate a design. They're like a combo of slices and save for web in PS. Using factories is infinitely faster than cumbersome layer comps, slice selections and export scripts.
Making things polished: Opacity lets me easily snap to whole pixels, instead of sub-pixel horseshit that makes everything look blurry. Using the pen tool in Opacity gives me smart, dynamic guides to help me align my anchor points.
"Layer styles:" Instead of confining me to one of every kind of layer style and forcing me into a specific compositing order for each, Opacity lets me do whatever I need to, in whatever order, as often as I want. It's liberating and time saving, especially when you're making UI elements.
The fact that it doesn't take a long time to launch is also nice.
Photoshop is bloaty and cluttered. There's no focus there and I'm pretty sure no one at Adobe is really giving any thought to how I will use their software. They're working up feature lists, and not much more than that.
(For those interested, here's Opacity, too: http://likethought.com/opacity/)
What, in particular, is well designed about Photoshop? What alternatives have you looked at?
My memory of it is that the features were haphazardly organized, all of operations required messing with horrible little mousy controls and it was obscure how to do anything beside spray paint on the image.