Spot-on aesthetics/audio and excellent gameplay. Check out the making-of to see just how much attention they put into making it feel NES-y: http://www.gamasutra.com/blogs/D...

As someone who didn't grow up in the age where these games were popular, I find Shovel Knight to be the perfect balance of hard and easy, where I don't get angry for dying so much. It's also a beautifully retro game with a lot of great secrets and a soundtrack to die for.