These ratings, if done objectively, can be a brutal exercise.
One thing I always try to keep in mind is that the marks should roughly reflect the outcome of the match.

Most seem to agree that 6 stands for an average performance. I just totted up my marks and they average out at 5.8 per starter - just below average, and that seems about right for a home game in which we failed to score in a hour and a half against a poor team that were there for the taking.
I think a lot of the numbers on this thread are too generous. The GK and back three gave the front of the team an excellent base to go on and win the game. They made a really bad job of doing that IMO, 4s and 5s are in order because some performances were below average, a couple well below.