I enjoyed seeing the labyrinthine old castle; it's an intriguing location, one I'd love to explore myself, doubly so with all the trapdoors and moving walls and so on. The plot with the Mane Six inadvertantly scaring each other was pretty good, too; amusing and entertaining, and I liked how it played out without any villains, heels, antagonists, or supernatural forces driving it. Just a bunch of very silly ponies. :)
And what's more, the episode didn't feel rushed. Quite the opposite, it flowed very naturally, which is all the more impressive considering there were several interwoven story strands that all had to be developed. The Simpsons writers are the undisputed masters of this, I think, but MLP:FiM proved that the Simpsons aren't the only series that can pull this off.
Twilight's squeeing (and wing-pomfing) when she first entered the old library was adorable. (I'm surprised the books were in such great condition after a thousand years of no care, though; for that matter, I'm surprised that Celestia never had them transferred to the Canterlot library.) All the others were very well-characterized as well, and they played off of each other very well; however, I think that Pinkie needs an episode focussing on her again, since her character is still a bit too heavy on comic relief and pure zaniness.
I wonder who the hooded figure in the end was. Judging by the cloak (?) and the glowing eyes, it might well be Zecora for all we know.
Oh yeah, and since I neglected to mention it last week: a slightly updated intro, too! Very nice.
Random bits: Twilight's good-natured quip that nopony likes sarcasm made me grin (as did Spike's preceding comment; I love his dry wit). It was also nice to see so plainly that Rarity's coat is indeed not pure white, but rather a kind of off-white. And finally... the star spiders were a bit of a red herring, weren't they?
Elsewhere: episode post in broniesaremagic; episode follow-up on EqD.