The fact that you can't do Contests online.
Seriously, that was such a wasted opportunity. Competitive Contests would've been an entire different class of playing Pokémon. Not to mention fun.
Wow, I hadn't even thought of that as a possibility! What a missed opportunity. Being able to compete in contests online against real people would have been so legit. I mean, they're fine as they are, but there would be a whole nother meta game surrounding it. I'm sure much much less popular than battling, but cool nonetheless. And I have a feeling contests won't be coming back. Even if they do in the inevitable DPP remakes, they will probs be in the DPP contest format, which I think is slightly inferior to RSE contests.