I've had some sense of Twilight's personal growth throughout the series, but it's mostly been in her ability to emotionally connect with others and actually give a flying feather about those around her. She doesn't let go of the little things well (and the attempt to give her that lesson in Games Ponies Play really felt rushed), her diplomacy skills don't quite exist yet (Baby Cakes), and there are two words for when her OCD completely takes over: Lesson Zero. They are not pretty words. We do not want those words to happen again. Especially at the new power level.

In my opinion, she was about halfway there -- but that's just it: half.

(This brings up questions about how Celestia and Luna reached alicorn status -- and still more on Luna's corruption. When you aren't in an emotional position to handle things yet and something influences you... Celestia's had at least a millennium to find her center: Twilight's had about a day. This would be a very bad time for a corrupting force to show up.)

something big and nasty is coming that Celestia and Luna will not be around to handle

Possible. The wedding episodes gave the series its Shane moment when it proved Celestia can be taken down by superior power -- and that superior power can exist. (Some people feel her backup plan there was 'Stall for nightfall and then you're in trouble.' It's generally agreed that under normal circumstances, Luna's nocturnal. Note her entrance at the end along with taking surveillance shifts earlier in the episode.) Taking out both sisters would be harder, but it's not impossible. And Cadence is in no way a fighter. Twilight's at least had a few hard-taught courses in battle tactics and desperation improv.

and whilst I know that our ponies are supposed to be grown ups they still behave a lot like children.

Word Of Faust is that the Mane Cast is mentally between twelve and eighteen: mature enough to handle a lot of things, young enough to have some childish responses. (Arrange them on that scale however you like: Pinkie's holding down the low end.) Physically, they're young adults. Of course, ponies do physically mature a lot faster than humans...

And y'know, it says something when the series has had sixty-five episodes and it feels rushed...

Oh, and as long as I'm posting: just spotted this.

http://i.imgur.com/Vn8l7QY.png

Derpy supposedly shows up twice after a season-long absence: in the opening and at the coronation, winking at the camera.