Although I've used gimp to do something quick, I've never really studied it. I'm still working on it, but here are a few features I've picked up that others might be helpful:

1. Ctrl+Mouse wheel: zooms in/out
I use "= - 0 1 2" a lot, but the mouse wheel is nice to zoom in and out quickly. I reset Zoom In to be = rather than + (Edit > Preferences > Dynamic Keyboard Shortcut), so I didn't have to use the shift. :-)

2. Edit > Paste Into and Edit > Buffer Paste/Copy
I haven't used these yet, but after reading about them I can imagine how I could in the future. I do a lot of cut and paste :-)

3. View > Dot-for-Dot
Will be nice for images designed to be printed.

4. Simple expressions or different units can be entered into numeric fields.
I won't change an image's unit anymore, but rather I'll leave the units as px and just enter inches and percentages.

5. Fuzzy Select Tool: If you click and drag the mouse, the Threshold will change --- left/up (decrease) and right/down (increase).

6. Ctrl, Shift affects the Selections Tools. There is a helpful tooltip at the bottom, but there is more to it. First, I found it necessary after clicking a Selection Tool that the image had to also be clicked before I could proceed. I'm not sure why. Now, if you click Shift/Ctrl/Shift+Ctrl BEFORE you click the mouse to begin your selection, the mode changes (add, substract, intersect). If you click Shift/Ctrl AFTER you click the mouse, an option of that particular Selection Tool changes (Rectangle Tool: changes expand from center). This is explained in the Gimp Doc, but doesn't sink in until you try it a couple of times. :-)

7. Selections can be turned into a Crop (Image or Layer > Crop to Selection).

8. A selection can be moved. If the resize handles are visible, just hold the left mouse button and move. If the resize handles aren't visible, click inside the selection and they will reappear. Or use Alt+mouse to move. In the past, I just clicked the Move Tool.

9. A selection can be cut/copied and pasted by using the mouse. This seems like a fast alternative to Edit > Cut/Copy and Paste as New Layer. Shift+Alt+mouse move: copy/paste, Ctrl+Alt+mouse move: cut/paste.