I am a man of simple needs, that's why for a long time my favorite image editing software has been... mspaint. I use it mainly for resizing images, adding text and highlighting a section. It has minimum clutter and is simple to use. But on my vista laptop mspaint started to refuse to open images prepared by mspaint itself! I searched the web and even asked for help on superuser. People told me to switch to Paint.Net. So I did.
* Good: Line angles can be in increments of 15 degrees (instead of 45 as in mspaint)
* Good: You can put arrows at line ends
* Good: You can have different line types
* Good: Crop to selection (instead of manually adjusting the size in mspaint which requires good mouse handling skills and sharp eyes to detect resizing handles)
* Good: Easy zoom in-out. I curse the designer of vista mspaint who introduced many zoom levels. It was much easier in XP.
* Good: You can write text even when zoomed in
* Good: Magic wand to remove unwanted backgrounds
* Good: Shows length of line
* Good: Can open multiple files at once
* Bad: It is not as easy as in mspaint to create a transparent selection for moving elements in picture closer to each other. In mspaint you just make a selection and click transparent background option. In Paint.Net you have to create a second layer with transparent background. Draw on the second layer, never on the background layer. With a single background layer, even the background will erase the underlying image:
* Bad: To move a portion of the image you first have to select it (shortcut S), then click move (shortcut M)
* Bad: Choosing an image quality option every time you do "save as" is annoying. It should remember your setttings and not ask again, especially when you just want to resize a couple of images.
* Getting used to: When you use the eraser tool, it deletes the background. You have to use the paintbrush with right-click (to paint in white)
* Promising: Rumor has it that future versions of paint.net might have batch editing. I would like to do multiple resizing (can be done with IrfanView).
Update 22 Oct 09: I also had a look at Gimp. It is a nightmare for a primitive guy like me. I couldn't even select a portion of the picture and move it. It took me 5 minutes to figure it out!