Yes, it's a paid app. No, it does not contain more features than the free Google Voice Callback app. The donate and free apps will always be the same as a tribute to the original open-sourced GV app by Evan Charlton.
Why the donate version? Because with just few cents more than a regular cup of Starbuck coffee (or just a fraction of what you may have saved from AT&T, Verizon, etc.), you can make me quite happy :) A college student can always appreciate some extra cash for coffee (and beer for appropriate occasions).