I guess another peeve would be the variety of blind bag ponies vs. the variety of brushable ponies.
This. Also, how FiM and the toyline don't match - Cupcake, Junebug, Sapphire Shores... Plus the fact that only recently did we start getting characters from the show, background or otherwise, as brushables. It took them 2 years to finally release Trixie? Really? And I'm still waiting for at least Lily Blossom to show up in the show!
No Starsong or Toola Roola analogues in G4. I know the Core 7 was a dumb idea, but come on, they adapted everyone else...
Lack of character continuity between Gens other than 3 (and even 3... Where's Minty? And Kimono?). Thank Hasbro for Applejack, at least...
Any kind of Gen VS Gen combat.
Some color schemes on G2 ponies. Dangit G2, I wanted to like you more! Why did you have such weird tastes in color? Why??
Knowing that certain characters are absolutely disliked by some people. *inner child speaking* Twilight Sparkle reminds me of myself, if you don't like her, does that mean you don't like me...? Sniff... -
Head/body mismatch. It should not be!
Mohohohooooooold. Also cancer.
Fading pink hair! Why is it so widespread? Why does it fade so easily? Why does it require such special treatment? Why did it have to be so pretty and have so many problems? Why, why, why??
...Can't remember more at the moment.