The number of episodes in a season doesn't really matter. It's the total number of episodes that makes a difference. Most sitcoms just go on way too long. Arrested Development was a show many believe was cut short too early (and it probably was) but maybe that was for the best as it kept the show fresh and never allowed it to stale (like it did in the recent revival).

The Simpsons is a show I think has been able to keep its variety over time. Sure, it's no where near as great as it used to be, but for a show that's 20+ years old I still find myself laughing at new jokes. Seinfeld is also a show I felt did not rehash a lot of material, but others seem to disagree. I definitely see this in shows like Raymond or Big Bang Theory, however.