This requires quite some effort guessing and learning what all the revamped icons will mean, for diehard users of older versions. It would be nice if the user could select between the old and the new icon set.

It would be better if CTRL+V duplicates CTRL+ALT+V's behaviour in this specific case, since it corresponds to the only menu item that is not grayed out in this case.

Having to use the "Canvas size..." dialog over and over again consumes a lot of time. There ought to be a way to directly use the mouse to do this instead of having to go through a popup dialog. After all, even Microsoft Paint has such feature.

Paint.net is just as easy and quick to use as Microsoft Paint (as opposed to Adobe Photoshop and Corel Paint Shop Pro), yet much more powerful than Microsoft Paint. Common actions such as cutting, pasting, cropping the canvas and drawing boxes, arrows and text are very quick and correctable(right before hitting the "Complete"-button/Escape-key) as well as undoable. This all makes digital life much easier. Unfortunately extending the canvas is cumbersome, which is almost the only downside I found in Paint.net v4.0.9 on non-Windows-XP-systems. The CTRL+V behaviour can confuse some newbies, thinking that the clipboard froze/locked-up, at least a simple reminder could be in place for them.