Paint.Net was only really open source entirely from 2004-2007 and partially from 2007-2009 (engine only, not the UI for the latter part).

My experience: Pinta is an inferior Paint.NET . It's about the same level of complexity, but everything is just like... 60-85% as good.

Krita is a heavier program, but has a similar level of polish. You might not use half the features, but Krita's what I'd suggest for anyone wanting an open source alternative to Paint.net