I liked how it began; a slice-of-life story, with winter approaching, Tank preparing for hibernation, and Dash apparently never having realized he'd do that. (So there's never been a winter yet since she got him?). More time could've been spent on explaining why she was so opposed to the idea; the focus was squarely on her reaction, while the reason she had it in the first place was just glossed over quickly.
The plot developed fairly quickly, but I thought the episode was going to take a turn for the worse when Dash said "I'll just have stop winter" — that's the sort of hare-brained plot hook that MLP usually and thankfully avoids. Fortunately it turned out better than I thought it might; Dash's attempts to sabotage the preparations were realistic (for a cartoon about technicolor ponies, anyway), and they didn't work out, which was realistic in itself.
Of course then she went to Cloudsdale and messed with the laboratory up there, and I was quite surprised that there were no (obvious) consequences. Dash didn't get caught; Cloudsdale didn't end up getting wrecked; and Ponyville didn't descend into chaos, either, beyond winter arriving very swiftly and suddenly (which actually saved the winter preparation team a lot of work and so arguably was a good thing). Nobody even appears to have suspected that Dash was behind the whole thing; one could interpret her sulking in her room as she having been given a time-out to reflect on her actions, but I don't think it was intended that way.
The resolution was also pretty weak. Dash couldn't accept losing a friend for months and dreaded being alone, but this was not addressed, and her friends did not help her in any way that went beyond "suck it up" (phrased a little less blunt, perhaps). Dash herself remarked on how her friends had helped her after crying for a while, but all I could think was — how had they? If Celestia'd appeared and demanded a friendship report, what would they have written? "Sometimes life is tough, so let yourself cry for ten seconds and all will be OK"? Now, allowing yourself to cry when you need to is important, but it doesn't solve problems, and Dash would not realistically have felt better from crying alone, and her friends really didn't do anything for her.
There could've been more of an emphasis on hibernation not being forever, on it being part of Tank's nature; on friends being there for you so you won't be lonely, on friends commiserating and comforting you in tough times; and for that matter on not trying to deny nature because it doesn't fit into your view of what the world should be like. Quite a few hooks for good friendship lessons, but they were all ignored.
That said, the episode wasn't all bad. Dash's antics and the lost opportunity for a good friendship lesson aside, it was pretty cute, as usual; the song was nice, and it had a number of funny moments, notably the exchange between Clear Skies, Open Skies and so on (reminded me of the classic "Who's on first?" routine). AJ's "crying inside" was funny in a wry sort of way as well, and then there was Spike's empathic "No" that Dash followed up with a "I'll take as a yes"; eminently quotable material.
I liked seeing Cloudsdale again, and was intrigued to learn it apparently moves — doesn't Dash live there? Getting another glimpse of her abode was also nice.
Dash's and Tank's slippers were somewhere between cute and stretching believability; Rarity's winter outfit, meanwhile, was squarely in "I need another mug of hot cider" territory, and I mean that in the best possible way.
Elsewhere: discussion in broniesaremagic. Episode follow-up on EqD. drhoz's review.