One thing I found really interesting about HGSS is the new Pokémon that get Power Swap via breeding. Now keep in mind that I'm talking about Power Swap here, the move that swaps Attack and SpAtk boosts and drops with another Pokémon, not Power Trick, which swaps the user's base Attack with its base Defense.
Here are the Pokémon that had access to Power Swap before HGSS:
Mr. Mime
Espeon
Mewtwo
Xatu
Girafarig
Medicham
With HGSS, five new Pokémon gained access to Power Swap:
Ninetales
Exeggutor
Tangrowth
Shiftry
Altaria
These five new Pokémon have something the six old Pokémon did not; they each have a move that lowers their own SpAtk by two levels when used. Ninetales has Overheat, Altaria has Draco Meteor, and Exeggutor, Shiftry, and Tangrowth have Leaf Storm. They can use their STAB 140-base power move, then pass the stat drop off to an opponent, recharging their own SpAtk in the process.
Now, I don't expect this to make a splash in Singles play. It's too situational. However, I think this could be useful in Double Battles for a few reasons.
1. With two opponents, the odds of one of them being a Special attacker is relatively high. The switches this will probably prompt can be leveraged into a significant advantage (more so than in Single Battles).
2. An Intimidate (or even two Intimidates) can be absorbed and passed off to an opponent along with the SpAtk drops. (Gyarados and Salamence, I'm looking at you.) If you can steal the Attack boost from a Dragon Dance while you're at it, then so much the better. Keep it for yourself, or pass it to an ally.