I wrote this back when it was leaked, and my impressions remain the same. Check 'em out below.

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What I liked:

1. Fantastic animation. The lighting and shadowing were great, giving the characters depth and atmosphere mood.
   
2. Excellent AppleDash chemistry. Not shipping, but their friendship and friendly rivalry played off each other. Chemistry at its finest.
   
3. AJ, your story's actually SCARY. MWAHAHAHAHAHAHAHAHA!!!
   
4. Good use of continuity. No true results, but at least the box wasn't left hanging unlike the rushed, lamebrain Twilicorn concept from last season. Hopefully, they don't drop the ball on it unlike last season.
   
5. Adorkable Twilight, but reigned in with more confidence, intelligence, and maturity. (Still, great for humor, bad for proving her worth as an alicorn princess. More development of her is needed to get passed this stage. She can't be like this forever.)
   
6. Twilight can fly well, hopefully ending the lazy retcon.
   
7. Stunt > Consequences > Resolution. Rarity got in over her head via her greed, scared the shit outta herself, and tore a piece of the precious royal tapestry.
   
8. It was just one hilarious Scooby-Doo parody. The ponies being so scared and running around like that gave me plenty of laughter.
   
9. Unlike Princess Twilight Sparkle, Pinkie wasn't a random dumbbell. She had purpose and a bit more intelligence to go with her randomness.
   
10. Nicely done callback from Lesson Zero.
    
11. Equal screentime for all of the characters, minus Pinkie. Angel's still a lovable dick.
    
12. The pacing is factually better in here. One of the premiere's biggest problems (and why PTS isn't so objectively good) is the pacing, forcing McCarthy to rush her plot development and insert contrived logic and plot devices. Here, there is a small DEM, but unlike Too Many Pinkie Pies, it fit the scope of the old castle and didn't really impact the plot so much.
    
13. Good cliffhanger. Now I wonder how it'll impact Season 4…if it will.
    

What I don't like:

1. Lay off the Spikabuse, pleeeeeeeeaaaase!
   
2. VERY predictable outcome. With Pinkie absent this whole time, it was obvious she was the master of the pipe organ. If the pony playing it was someone else (like Celestia, Luna, or a true ghost), then that'd be a good twist.
   
3. Fluttershy was caring for Angel, but still behaved like a rather gigantic scaredy-cat. (This case, though, it's somewhat justified given the circumstances.)
   
4. Twilight showing VERY SLOW development as an alicorn princess. She was funny, but keeps proving how pointless her ascension is so far. I already linked what kind of development Twilight should ger to prove her worth.
   

Altogether, I currently really like it, and it's so far a MUCH better episode than PTS. A good debut episode for Josh Haber. ^__^

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Source: S04:E03 - Castle Mane-ia