As a pegasister, I've seen ever My Little Pony episode :). I didn't get the DVD, though (this episode was a two-part episode in season 2). I really liked this special. The plot was bit more complex, and the plot twist was interesting. I don't think, however, kids are going to understand how Twilight feels when she discovers her brother is engaged. But this is kid-friendly, and I'm sure kids will enjoy it.
There is some violence; the main villain may be scary for young kids, and her minions attack Canterlot. There's also some romance, since this revolves around a wedding, and there's a kiss. Of course there's consumerism, and the entire show could be seen as an entire ad for the My Little Pony merchandise.