What is exactly meant by website on-site over-optimisation post penguin?

I know what the term means by I'd like to know how to draw a line between an over- and under-optimised website. I've registered an account with seomoz recently and followed their guidelines to get an A-grade for both my homepage and single urls. I'm a bit worried in case my website is over-optimised now.

Thanks!

Common question, but so many posts on this topic already...you should be able to find some via the 'search' function