Exploration - especially as a major point in the game - is a terrific kind of thing that makes PGC very valuable in concept; and fundamentally it's a big building block of games aforementioned. It's the distribution - and actual presence - of gameplay elements that really determines the overall funness of PGC.
Yeah, this.
A problem I'm dealing with now is reward for exploration. People don't explore to find yet another dead end. Everyone wants to find something special. That's the holy grail. Lot's of special things spaced out in a way that is unpredictable so that it drives the player to explore and be pleasantly surprised at reasonable intervals.