You really aren't missing much. I've gotten all the stars before (numerous times actually) and the novelty of what happens once you do get all the stars wears off pretty quickly. You'll soon be starting a new file over so you can have something else to do. But hey. It's your goal.
I personally would suggest that you try getting all the stars in Super Mario Galaxy. At least there you can...
SPOILER!!:
I have yet to do that. Luigi's damn Purple coins are tricky to get.
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