I just want to mention that I don't want to make Paint.Net a Photoshop clone, but these are some functions that programmers should implement in Paint.Net.
-Professional color adjustment (exposure, contrast, temperature, tint, sharpness...)
-Professional selection tools (Magic Wand based on more criteria)
-More brush types (Native Windows 7 Paint has more!!!)
-More layer types and a possibility to group them
By the way, Paint.Net should have:
-Adobe RGB support
-Ribbon design (like Office 2007)
-Possibility to send photos to popular sites (Flickr, Facebook, Mobile Me...)
-Add-in management (a easy way to add or remove plugins)
Anyone wants to add anything?