99% of the time, I just need to crop an image to a certain size. GIMP is actually much better than photoshop for this particular exact task. Nice UI for resizing the selection box in place. Other simple tweaks - brightness/contrast etc - are handled perfectly competently.
It certainly isn't a drop-in replacement for the sort of magic 'proper' photoshoppers do. But for a semi-competent web-monkey like me, it suffices perfectly well on those occasions when imagemagick is too blunt an instrument.
Vector to bitmap conversion sucks, though. Seriously, if IM can do this without pixellating everything to buggery, why not GIMP?
Lightroom is far better for the tasks you mentioned than GIMP. IrfanView or XnView would be also perfectly adequate for your simple tasks.
Forget about easy skin retouching in GIMP, no 16-bit/channel in official release yet, forget about high-end fashion photography with GIMP. It's still not there :-(
Well, you can change that in a way. Just use the money you would otherwise pay Adobe and fund the feature you need. You can easily contact the developers, there's a list of them here https://git.gnome.org/browse/gimp/tree/AUTHORS ; By browsing the source code, you can also find their emails.
I would actually pay 50€ or so for Pixelmator if it'd be available for Linux – but sadly it's not.
I'd think, though, that the Pixelmator devs could port it to Linux quite easily, as most of the code already works with OpenGL/OpenCL/etc. so they'd only have to port the Cocoa-based UI to QT.
Edit: Answering my own question... Not the main repository, but it's something.
http://sourceforge.net/projects/gimp-win/files/GIMP%20%2B%20...