Making it so text is not rasterized / pixelized right away, letting you easily change the text later on.
But still leaving text rasterization as an option.

We know it's reverse engineering, but it's a feature that is crucial.

8-bit PNGs with alpha transparency are images that can contain semi transparent pixels, unlike regular 8-bit PNGs. The main advantages over 32-bit PNGs is that they take up less space, and degrade more gracefully with IE6.
Few editing tools support them, for some reason.

More info:
http://blogs.sitepoint.com/png8-the-clear-winner/
http://www.communitymx.com/content/article.cfm?cid=E4024

We need to be able to resize and rotate individual layers

cursor of mouse must be changed when the drawing tolls is different. and the size of cursor related size of pinceau (désolé je suis francais)

This will let you more easily select a wide range of colors instead of having to mess with the palette size.

Like it's in GIMP. Open gif's and see the layers, save as a animation (gif)

Every time a picture is saved as JPG, the user needs to change its quality again.

In Paint.NET and other programs you can hold CTRL (or shift in GIMP) and click multiple areas of an image with the magic wand in order to select the desired portion of the image. This is particularly useful when trying to make an image have a transparent background.

As in Paint.NET, by moving mouse while holding right mouse button.

Add printing option to Pinta like File | Print.
More info at bug tracker: https://bugs.launchpad.net/pinta/+bug/728426

being able to open paint.net files and psd files in Pinta would be extremely useful

When selecting gradient a second option to use transparency mode is available to allow gradient from solid colour to transparent. Really usefull tool from Paint.Net

Add ability to use pressure information from Wacom tablet pen.
Useful for Wacom graphic tablets and TabletPCs.

Exactly like described here:
http://blog.getpaint.net/2012/04/08/new-features-for-paint-net-4-0/ in the "Antialiased Selections" part. Would be awesome!!

when you use the tool line you can bend it in several places