You lose the n capablities of the g device.
Which is pretty obvious, you cant utilize new technology to its fullest if one side doesn't have it.
Its the same thing as g and b guys. You can use B devices with a g router, just not to as great a speed or distance. You can use all g and b devices with n except the device wont be as fast or be able to go as far back. The router doesnt change. If you had a g laptop and a n laptop. The n would get more coverage and faster speeds, but it wouldnt get the same as g just because its present.