When given the idea of 'small is beautiful' - I'd also say small is easy to use.
I keep going back to Enlightenment, and to WindowMaker - because they are small and simple, and easy to use. Which is ironic, because windowmaker is outdated in terms of looking shiny, and Enlightenment at one time, was known for being a pig and clunky, though it was beautiful visually. Now enlightenment is amazing - it is simple and fast, and beautiful.
When confronted with Unity for the first time, I thought 'This sure looks neat' but then became baffled at the inconsistency of it'. Compared to older window managers, I find it extremely convoluted to get simple tasks done. It has so much going for it but is grossly inconsistent.
Though it may be hackish, I think compositing plays a heavy role in user adoption - it ads sex appeal to the interface. From what I understand, it has some useful benefits as well, like preventing one hung window from jamming the whole interface.