E3 2015 has been a whirlwind of events, and we've had no shortage of new gameplay footage from Yoshi's Woolly World. The most recent gameplay video from Nintendo Treehouse Live @ E3 shows more of the co-op mode, as well as some of the strategies to use while collecting items throughout the levels.
Collectibles are the main feature within Yoshi's Woolly World that create the challenging aspect of the game. Sure, you have to know how to beat a boss, but collectibles are hidden in many clever places that will take you extra time and thought to discover. The other highly mentioned feature within the video is the Amiibo functionality. In co-op the second player always stays as the red Yoshi, but player one can customize their Yoshi experience depending on which Amiibo they have.
Source: Nintendo Treehouse Live @ E3