Nintendo just wants you to know this: it launched the 3DS too early without fully considering the quality of its software support or the attitudes of the market it was hoping to win over. The 3DS price drop to $180 made that clear and the Expansion Slide Pad add-on rams the point home. The 1500 yen (~$20) peripheral attaches to the bottom of the 3DS, adding significant bulk along with a right-side analog pad and new triggers, and runs off a single AAA battery. That battery allows the Expansion Slide Pad to talk to the 3DS via IR.
While the second analog slider has "Monster Hunter" written all over it, a number of other games will also support the accessory. Given Nintendo's history of product revision with the DS line and the fact that several games are already adding in optional support for dual analog controls, a new model 3DS seems destined to arrive sooner rather than later. With better planning and a delayed launch, the second stick could've been on board from the very beginning.