I think the sad thing about MMC is that the big surprise was ruined due to the
Merchandise Driven nature of the show. The only real surprise the episode had was the celestial plane which was pretty much the only thing that wasn't in promotional materials in any capacity. Even the fact Celestia was going to sing was spoiled a few days before.
Compare the Season 2 finale, which had a lot of hype and build up, but there was still a big twist in the episode that no one really saw coming; the Changelings. There was like one news article that most of the fandom didn't notice that mentioned it, but they didn't post a video or anything. All the focus in the promos was on Cadance & Shining Armor and the wedding itself, and in hindsight I realize why.
There are no changelings in the Royal Wedding toyline. We're only
now getting Queen Chrysalis toys, most likely due to
popular demand, a year after her debut and long after the Wedding line ended.
Granted there aren't really any new characters to make toys out of from MMC unless you count the blonde hoofmaidens that show up for one scene. Now that I think about it, why isn't Hasbro making toys of them? Their one role is to sing how great the Twilight Princess is!
What I'm trying to say is, continuing this trend I think the only really big surprises from the show finales will be stuff Hasbro
doesn't intend to make toys of (at least not right away during the appropriate toyline).
Oh and for the record I liked the Season 3 finale. It was a musical! It had feels! Its hard for me to hate on it even if I think it should have been a two-parter.