i like the idea of controlled customisation too. i think what you are trying to get at can be explained in a well thought out metaphor =).

Don't give people vanilla icecream (plain like facebook) but don't give them neopolitan like myspace do, perhaps just vanilla with little choc chips in them, so it's not plain, there is something awesome in there to keep the visitors entertained, yet they won't throw colours everywhere with there multi flavoured dessert