I didn't think much was going to come of this episode. I simply thought it would be a spoof of a monster movie, and later on it was, but it delivers so much more. Once again I underestimated this show, but I've grown to love the feeling of having my expectations exceeded.
In this episode we learn that the greedier a dragon is, the faster he grows. Spike isn't usually a greedy dragon, though... he even gives Rarity a priceless gem that he was keeping for his birthday dinner. But today is his first birthday in Ponyville; he usually gets only one crummy present from Twilight but more friends means more gifts. Spike decides to take this to the extreme, demanding gifts for the sake of hoarding. Later on he resorts to outright stealing, but wait a minute, now he's triple his size and no one knows why!
Well, except for Zecora who tells Twilight that dragons are naturally greedy and encourages her to contain Spike so he can't steal anything right after Spike steals everything from Zecora's hut. But Twilight can only keep Spike in the library for so long, and from there Spike becomes a giant rampaging hoarder.
But the Aesop about not being greedy isn't all the episode has to offer. Remember how Spike has an obvious crush on Rarity? The subplot of the episode focuses on the development of their relationship; after Spike gives her the gem, Rarity starts acting very affectionate towards Spike, kissing him, nuzzling him, and calling him pet names. Seeing some actual development from what was previously a one-way romance is quite touching, especially the parts near the end.
I'm actually hard-pressed to find criticism, except one problem I can't get past: Twilight has known Spike for all his life, and yet has not researched anything about how dragons mature or grow. This has been beaten to death on the discussion page though, and considering the episode's subject matter, is largely irrelevant.
This episode cast Spike in a new light for me. Beforehand I thought of him merely as a dutiful kid assistant reliant on Twilight with a dash of eccentricity, which wasn't particularly bad, but with this episode I can fully embrace him as a well-thought out main character. The ending My God What Have I Done
moment and Rarity pulling him out of it sealed the deal for me. All in all, I thought it was a sublime episode.