The 5/5 ranking seems to be the standard, but it doesn't really make sense. most players give a very high 5 ranking to a normal player and a very low one to someone that pisses them off in some way. Therefore outstanding players aren't distinguishable from normal players. i think a 4/5 ranking would make more sense if nothing extraordinary happens.
I think the best suggestion to counteract this has been that every time someone DOESN'T rate you you automatically get 3.0 stars. This would go a long way to enforcing 3 as average (ie the way the current system was intended to be used).
As the playerbase begins it's decent from 4.8 in time it would also grow more acceptable to leave 3' or 4' star ratings for average/good play.