Characters that have grown on me:
Applejack: Talk about underrated. However, the worst part of her underrated status doesn't come from the fandom, it comes from the creative staff. They seem far too willing to keep her within the "Family, Apples, Sweet Apple Acres" comfort zone, and the character needs to have stories beyond that. The Last Roundup could've been a great episode outside of this, but Dərpy, Pinkie Pie and a paper thin focus on her, put the nix on that. Praying that Season 5 puts her in a different situation.
Characters that have done the opposite:
Cadence and Shining Armor: Talk about a badly introduced good idea, that hasn't paid off in the slightest nor do I expect it too.
Twilight Sparkle: Speaking of bad executed idea. Her Alicornification and everything surrounding it has rubbed me the wrong way. If it weren't for TT123, she would've spent this season being little more than a paragon of virtue that's there to hand down lectures. Even then, there's still everything that's happened this season and last that has made this character less likable than before.
Pinkie Pie: Speaking of less likeable, Pinkie Pie has gone from being a character that was funny, to a bad joke machine. Worst pony is Worst Pony.