alot of you have said that twilight doesn't have the right temperament to be a leader, and that's pretty true.
but since equestria is (pretty much?) a kingdom, ruled by a princess, an absolute power so to speak, that's exactly the kind of leader you'd want to take over the throne.
a dictatorship can be ok as long as the dictator is benevolent. an absolute ruler that doesn't want to rule won't go power hungry, and a ruler with not much charisma or confidence would be much more inclined to do what's best for the country instead of themselves. twilight, i dont think, would ever go into POWER HUNGRY MODE. i think she would always stay a very humble pony who tries to do what's best for equestria, and the fact that she can always listen to her friends' advice is a really good trait in a leader as well.
i think her biggest problems would be making public appearances, foreign relations (because she's kind of a weird spaz sometimes lol), and being less prone to getting stressed out. when times are tough though (crystal empire, for example), she's always been very competent, and she doesn't let the stress get to her, and she's always got what it takes to solve the problem. it's only when dealing with deadlines or other ponies that she has issues.
i'm not sure if celestia is grooming her to lead...i can see it, and i've been wondering about it a little myself. also, celestia lets twilight stay in luna's room! maybe if luna ever goes off the deep end again and becomes nightmare moon, twilight would take her place?