Unexpected behavior on new and imported images
I'm not sure I have the correct terminology, but the biggest problem(s) with GIMP adoption is how it behaves right when you open a new image to start doing stuff.
Expected: click pencil tool and start drawing all over the image. This is what happens in every graphics program I have ever used, EXCEPT the GIMP. Actual: often (but not always) nothing. Make a selection or new layer, then select pencil, then it draws on the image.
This is true for paint bucket, pencil, text, etc.
I don't know if the default concept of "selected area", "selected layer" or what is responsible for the unexpected behavior, but fixing this default will make GIMP infinitely more user-friendly.
If I open an image, click text tool, and type, it should automatically select the whole area of the only layer, and the text should show. User shouldn't have to create/select layer, nor select area, nor drag the text field bigger to see typed text add to the image. It needs to just happen.
Another less-important, but still vital, behavior is to make a rectangle draw tool. When I have a color selected, I should be able to draw a positive (colored) rectangle with it. Users should still be able to do things the slow and unintuitive way, by selecting a rectangular area, and filling that area with the paint bucket tool, but let everyone coming from any other graphics program do the normal thing. Select a rectangle tool, and click and drag to position and size it. Color is whatever user selected on the palette. No secondary step of filling required.