Can anyone explain the effect of worker difficulty?  I see where it asks the expected hashrate of the worker and then raises the difficulty for that worker if the hashrate is very high.  What effect does this actually have?  What effect does it have on a worker to have too high or too low of a difficulty set?  I googled but could not really find it explained.  If someone has a link or an explanation I would appreciate it.

It means that you aim for a share with more worth. E.g. a share at difficulty 1 is worth 1 share, 2 is worth 2 shares.. and so on. The difficulty refers to the target set for your hashes. At difficulty 2 there are half as many possible hashes which would contribute a share when compared to difficulty 1.

The reason it's useful is that with a very high hashrate (>2GH/s) you'll find so many difficulty 1 shares that it'll use up a lot of communication and it floods the server with requests.. so if you look for more difficult shares, you still (theoretically) get the same rewards, but the communication with the pool server is lower.