I miss trainer animation sprites from Platinum to Black 2. I also miss when gym leaders, villains, and Elite Four provided commentary when their Pokemon is hurt or when they're down to their last Pokemon.
I also missed that awesome theme that plays in Black/White 2 when you face the Gym leader's last Pokemon.
Having the item finder on the bottom screen like Black/White 2 was awesome as well.
I really enjoyed Black/White 2's Black Tower or Tree challenge. Great place to level grind. Having a difficulty setting was great too.
One thing I missed in fourth gen was the underground bases. We're sort of getting that in OR/AS though.
Oh and I hope the Soar feature is staying for future generations.