Lines need to be rounded on ends. When lines meet they come to a sharp connection.
Line weights need to be unnoticeably different.
High contrast should not be noticeable within the icon as a whole. Together the elements of the icons should compliment each-other, not compete. Keeping icons simplistic.
Elements that are filled entirely should only be used when necessary and used as accent pieces, should not take up the icon entirely.
Icons should be able to work in up to three colors but also work in one color.
Icons utilizing type should keep type limited.
If type is utilized it should be in Open Sans Bold and tweaked to follow line characteristics in regards to rounded edges.
in collaboration with Cory Hartline.