Spark Core Shield PCB design review
You've got lots of little proximity issues and shorts that Eagle's DRC should have pointed out to you. Did you run the DRC? When you do, it's best to use the design rules provided by the board house you plan to use. For example, here's OSH Park's design rules (scroll half way down the page). They even provide a .dru file to use directly as your DRC.

I've circled in orange the problems that I could see. As whatsisname suggested, it'd be more helpful if you provided the schematic as well.