Just switched distros, and my panel (which used to be transparent) is no longer transparent. I tried mucking about with the themes, but no matter which one I choose the background remains decidedly opaque.

I do actually have compositing enabled. Other transparency effects (konsole background, transparent windows on move, etc.) work fine.

Thoughts?

Have you ever applied a custom Plasma theme?

Can you reproduce this under a new user?

Hah, apparently I'm just an idiot. It's transparent, it's just not as transparent as it used to be. The new theme makes it much darker. Sorry for the false alarm...

Your desktop wallpaper can also have a significant impact on how transparent/translucent the panel appears to be - darker wallpapers often make the panel appear more opaque.