What I liked:

1. The battle. Seriously, the best part of the finale and quite convincing.
   
2. Establishing continuity from It's About Time without being rammed over the head.
   
3. In some capacity, addressing the elephant in the room: Twilight's lack of role as princess throughout this season. And the fact she has a title and new home gives her some purpose. Maybe come season five, the writers won't ignore it. (Come the season review, I won't give DHX a pass.)
   
4. Referencing the keys and using them (plus referencing the bad key episodes like IAEBB and Rainbow Falls and other dumb episodes this season like Equestria Games).
   
5. Discord's hilarity from Part 1.
   
6. The songs are decently composed. (Not a fan of the line, "Your destiny's uncertain" or its double entendre: understandable in full context, but semi-retcons her purpose revealed in MMC.)
   
7. The fact that the key is Scorpan's pendent, which Discord gave. Reinforces the good Scorpan wanted to give.
   
8. Discord's now an official friend of Twilight, and I chuckled when he gave flowers to Celestia sincerely.
   

What I didn't like:

1. The third act of Part 2 really raced through and felt rather stilted.
   
2. The main problem from this season reared its ugly head again here: WAY too much exposition.
   
3. How the AliTrio never went out to completely erase Twilight's presence. Yeah, it lead to the epic fight, but it was too contrived.
   
4. The designs of the Rainbow Power ponies are WAY too busy and rely on style over substance.
   

I don't know if I'll call it good, but it's definitely a two-parter I like so far. I'll have to re-watch it once more to make a better analysis.

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Source: Unofficial SEASON FINALE Discussion Topic - Twilight's Kingdom