Using the ratings of episodes in IMDb this graph making doohickey by Kevin Wu will show the relative quality of seasons of your favourite shows. Look at that fourth season of Community, for instance!
Though individual seasons of The Walking Dead have risen or fallen, the average seems to have kept steady, but look at season 4 - it contains both the highest and lowest rated episodes, by quite a margin.
And Game of Thrones has developed a characteristic ramping up of quality along with tension as each season goes on. Lets see if the upcoming season fits the pattern.