Basically, for most people this means you will get half the miles you currently get. They are basing it on ticket price, not distance. Maybe it isn't as bad for those on short flights but it is definitely a decrease all around.
I heard Delta also went with a similar plan? They list a calculator on the site. The change doesn't go into effect until March. Although United is my preferred airline, I don't see a reason for it to be after this change.