I don't think this has been pointed out (if I has I apologize), but one thing that bothers me is the following.
Becoming a Princess in Equestria is apparently very difficult, as Celestia said, it hasn't happened in over 1,000 years. However, Twilight achieves this before any of her friends achieve their 'goals' or 'destinies', which seems very odd.
As an example, Rainbow Dash has worked to become a Wonderbolt for, at least, a good part of her life, Twilight has (consciously) worked to become a Princess for none of her life, however she achieves her 'destiny' before Dash.
This leads me to one of two conclusions:
-Becoming a Princess is actually easier than becoming a Wonderbolt.
-Twilight is just that much better at life than Rainbow.
Since the first is disproved in-show, that only leaves the second, which I find quite depressing.