I'm honestly curious: where do you think the boundary between work and play occurs here? It seems to me that many of those that make those mods do it mostly as a recreation, don't you think?
Exactly! And that's the beauty of designing such a game in the right way. There's an opportunity to narrow down the freedoms in just the right way to limit failure rates.
I don't think complexity alone is a game stopping issue. There are plenty of highly complex games that people find fun and addictive. But complexity in combination with high failure rates (a lot of trial and error) will kill the fun.
But I think that high complexity with low error rates is perhaps exactly what makes a game fun. If you can express yourself freely and roam around while expecting most of your actions to produce an interesting result the game will be very interesting.
