Fan service in this context, in case you are unaware, is when a TV show or movie pays some sort of tribute to their own fans. My Little Pony does this very frequently. For example, they have made references to bronies in the song “Equestria Girls”, given Derpy Hooves a speaking role, and actually called her Derpy (which we will come back to), and much more. In addition, the MLP staff has released rumors that some of the background ponies (that bronies love) are going to have bigger parts in the upcoming season.
However, I feel that sometimes the fan service can be too much. Derpy is a great example of this. When she was featured in an episode (see clip below), there was a public outcry from parents of children who watched the show, saying the name Derpy, combined with the voice, was offensive to those with mental issues. In response, her voice was changed to sound, well, less derpy in later recordings. This proceeded to upset all of the bronies who liked Derpy’s original voice and who didn’t believe the name Derpy carried negative connotations in this case. The bronies loved her; they didn’t love laughing at her.
I think MLP went a little too far with the original footage. While I enjoy fan service, it shouldn’t seem like it is coming off at someone else’s expense. The way the scene was originally done, it was easy to take offense at and it seemed insulting to those with mental illnesses. I know that no one meant any harm and MLP writers were only trying to pay homage to their fan base, but from a perspective outside that fan base, it just came off as rude.
The instance above was just going a little far. The Legend of Korra, on the other hand, beat the idea of fan service over the head with the voice of General Iroh. He has the same voice actor (Dante Basco) as Zuko, the main firebender from the original Avatar, and General Iroh’s grandfather. This was probably done to pay homage to the fans from the original series and pay tribute to the previous series. However, since I literally watched Avatar and then immediately picked up Korra, I knew instantly that General Iroh and Zuko had the exact same voice. Not a single difference. For me, having the two characters depicted by the same voice actor would have been enough, but giving them the same voice was overkill. It also made it very difficult to figure out how old General Iroh is (I actually have zero idea). If Basco had changed his voice slightly, I would have 99 problems and Iroh’s voice wouldn’t be one of them. But right now I just have 100 problems.
In short, fan service is awesome in small doses. In big ones, it just makes people upset and sometimes angry.