After watching this episode, I'm trying as hard as I can to find anything to increase my opinion of it as much as I can. I didn't -hate- the episode; that title is reserved for ones I would actively skip when I rewatch the show, but I can't really like it either. I'm gonna get the negative out of the way first, so I can leave the post on a positive note.
My primary problem with it was probably how over the top RD does everything over the course of the episode. Yes, I get it, her pet has to leave for a while, and she doesn't want to accept that. I also get that this is a cartoon, and such over the top stuff is normally used for entertainment value. But I didn't feel entertained by or sympathetic towards any of her actions. I probably mentally facepalmed more than anything.
Tank's got to hibernate. Fluttershy has explained this. Spike has explained this. He's gonna be back. That alone is enough to remove any empathy from me towards the idea of it being some big loss. Sure, I look forward to doing things with my friends, and it sucks when one has to move away for a while. But when you know they'll be back, it's not goodbye forever, it's see you when you get back. I also understand that I'm nothing like Rainbow Dash. I'm very level-headed and take everything from a logical standpoint. I feel emotion but temper it by understanding the reason behind the feels*. But you shouldn't have to think like the character to be able to sympathize with them in an episode - especially when that episode is meant to draw your sympathy as a generic viewer of the show.
Her emotional reaction makes sense; as others have pointed out from Mare Do Well, she hates being alone - and Tank's her only companion we as viewers know of when she's not with the other mane 6. But her physical reactions were vastly in excess of what was actually going on and made her seem like a flanderized villain. I'm sorry, but everything she did was an excessive overreaction for the sake of comedy, and it just didn't work for me. Without the aforementioned sympathy for her problem, all of her reactions were groan-inducing. The setup was fine, and I liked the ending with her sitting with tank, but the journey from that start to the finish was painful, to the point where I was actually glad when Fluttershy finally stepped in and laid things out bluntly near the end so it could all stop**. I even enjoyed her song. But on reflection, the way she goes along so happily singing about this scheme she's got to stop winter makes that grinch face so appropriate.
That said, the main things potentially pulling this episode up from a dislike to a meh. I enjoyed all the jokes as usual. The who's on first? gag, the angry face, the grinch face, pinkie pie walking through the cloud wall of Rainbow's room instead of the door, Applejack as a whole (for what little she did), and the aforementioned Fluttershy drawing from her breezy lesson and laying down the harsh truth to Rainbow. These were all great. I even had to sigh a bit (in a good way) when Winter is Coming was said. I also enjoyed the world building of how Cloudsdale is this floating city that travels around the country to deliver weather to areas that are due for it.
Basically I enjoyed everything that wasn't the way the centralized plot was executed. I'm no story writer, so I'm not gonna start throwing around suggestions about what they -should- have done. The episode's done, and the best I can muster up if I think about all the angles is a rousing "meh." The most I could say on an improvement is I could believe the reaction if for whatever reason Tank was leaving for good. Not that I want to see that happen, but it would have at least made the plot of this episode feel a lot less contrived.
As to the continuity "error" some are raising, to me a continuous timeline in episode sequence goes out the window as soon as Fall Weather Friends happened two episodes after Winter Wrap-up. And even if this is all in order, as others have said, it's entirely possible she spent other non-episode winter(s) with other pre-planned events and this is the first winter she has with plans for Tank.
*Yes, I have experienced loss, for those of you who might try to pull that card. I had to put down my childhood pet not two years back.
**For those of you throwing this ad hominem around, I'm not one of those people who legitimately feels making Fluttershy cry is inherently a bad thing just because she's Fluttershy. I'll play along with that line of thinking, because it's fun. In reality, it's not cool to go out of your way to make others cry. But it's all based on context. In this case, Rainbow needed this to get her to let her feelings (and tears) to flow, so that she could come to terms with what was happening. And after everything that was happening, it was just so satisfying to see somebody finally bring her to accepting the truth whether she wanted to or not. The crying itself had nothing to do with my enjoyment of the scene.