The one problem I have with this episode is a real nitpick that really is insignificant compared to the awesomeoness of this episode:
The fact that Twilgiht's test of her element is giving up the alicorn magic for her friends. If she hadn't recieved her key for that, Equestria would have fallen under the control of all-mighty Tirek, with no way to defend themselves with even alicorn magic being done away with.
And also, Twilight wasn't really giving up something of hers. Maybe if (somehow) she had to give herself up, be a slave of Tirek or something in exchange for the safety of all of her friends. But nope. She gives up the lives and well-being of all of flipping Equestria for her friends.
I guess you could argue it's the ultimate sacrifice and shows her love for her friends, but I'm still kinda irritated that she'd let Equestria fall under Tirek's control just for six ponies, even if they mean the entire world to her.
Again, it's not that significant, and there may be other ways of looking at it that I fail to see, but other than that, I loved this episode.