I think this is a key question for any 3d printer / software setup. Most of the posts on Slashdot seem to center around "designing something for real" (prototyping, replacing a part, etc.)
But I think a recreational version would take off if done right. For example, my 7 year old granddaughter loves minecraft, and spends hours building things there. I think she would love the ability to print out stuff she has built there. She also likes to make her own videos. She will arrange her dollhouses and stuff animals and make up a story involving them, and record it. I think she would love the ability to design her own dollhouses, sets, etc.
For her, a minecraft approach of dropping and destroying pre-made blocks, etc., would work very well. Especially if she can paint and color her model of whatever after it is printed. She won't care about the exact dimensions, etc., as long as it fits together. Let the software handle that.
So the problem becomes, I think, "know the audience" and design appropriately for that audience.