You want to know why? Well:
-Mediocre two-part special with a crappy villain.
-No Rarity episode.
-A bullying PSA.
-Not enough emphasis on friendship like in Seasons One and Two.
-The possibility that Twilight will become an alicorn princess.
The writers of the show better get their acts together for Season Four. They know they have a fanbase in the form of us bronies and pegasisters. If any time the show is starting to suck like right now, they should be listening to us--their fanbase--so that they could fix any mistakes they've made in Season Three so far.
The reason why most other shows jump the shark is simply because the writers never listen to their audiences. Us fans may not own the rights to those shows, but at the same time we have as much say as to what should go in them as the writers themselves. This was why BioWare compensated for Mass Effect 3's terrible ending with the Extended Cut, and it was also why Hasbro (the people who did MLP) resurrected G1 Optimus Prime after all of the backlash toward Rodimus Prime. If Hasbro has no idea what to do with MLP, they should turn to audience feedback, just like they did regarding Optimus and Rodimus, and just like BioWare when they released the Extended Cut endings to Mass Effect 3.
It's not too late to turn to the audience for help and feedback. But, if they don't listen to us bronies, we might as well move on to something better, and they'll lose a lot of money and ratings for it in the process, which can be harmful to them!