Adobe Illustrator is not a paint application. It's a graphic design application.

Corel Painter is a digital paint application. [0]

Usage differences between these two programs is huge.

Usually paint programs lack filters, but specialize in brush macros, biasing behavior in favor of starting with a blank drawing areas.

Meanwhile image manipulation programs pretty much have a configurable airbrush and replicate some dark room techniques, and possess a rich filter library, biasing behavior in favor of altering and layering existing content together, after the original creative process.

[0] http://www.corel.com/

While I'd agree that GIMP and Krita are not same, I think Krita is still closer to GIMP than Illustrator, Ai being a vector editor.

Meanwhile, though probably known to most of the HN crowd it's also worth mentioning Inkscape which is a FOSS alternative to Illustrator of good quality.

I agree with that. Some things are a bit different, like PS -> GIMP, but once I had the basic equivalents down, I found it a great tool.