Instagram. I'm not big on it, but I sure know a lot of folks that live/die by it. A friend of mine was asking me what was so good about it and after telling her, she liked the idea, BUT she's big on iG and has tons of photos on it. She is definitely in to the social aspect of it as well. I got my nephew on to WP, but he is tired of looking for his iPod Touch to get/use iG.
Pandora. Even thought it has been said that it'll officially come, it's not here yet. Until then, this app, regardless of how good Xbox Music, Nokia Music & Spotify may be, people still have their stations and enjoy using Pandora.