I've never been a super grindy, max stats kinda guy, so "OMG INFINITE VARIETY OF WORLDS TO EXPLORE AND ALSO IT'LL HELP ME ROUND OUT MY COLLECTION" is more appealing than any particular level of challenge, and I was never uber enough to steamroll everything anyway.
I feel like it had more to do in post-game than others, too, but I could just be making that up.
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http://www.brawlinthefamily.com/images/Boomganon.png
Dang ol' BlAcK TuRtLe man tell you what