For instance Gimp also doesn't have "smart sharpening", a very useful filter that Photoshop users will recognize immediately.

Applying this filter in Gimp is more complicated, involving several steps (do a search on Google for tutorials), but in the process of applying such filters you'll learn a lot about image composition and about various tricks you can mix and match for other effects.

And the nice thing about it is that Gimp is so extensible that you can write a plugin for it, so you don't have to repeat those steps every time. You can do so with Python or Scheme. Heck, if you search around, somebody else probably wrote a good one.

So when using Gimp, don't expect a Photoshop clone, because it isn't.