Probably the nicest thing about X-bar theory is the fact that it is so neat and tidy.
All the categories take complements, just as verbs take objects as their complements. If these neat and tidy facts all come as part of Universal Grammar, it might explain how we are able to learn languages so efficiently as children. But look at the subject she. Why is it hanging off from the top left of IP? Do only IPs have that? Perhaps other categories have the same kind of thing happening? That would be really neat! What about a sentence like this?
She quickly punched him.
Where does quickly go?