I couldn't help but think of Charmed.
Though in some cases, that might be wrong. NCIS is consistent every season and House for that matter. Hell, Damages seemed to have gotten worse in its three seasons. In some rare cases past the 5th season
In most cases it is correct however, on occasion, there may be a sudden up kick season that makes everyone go "ooh! It's actually quite good this season".
when i read that i couldn't help but think of Buffy the Vampire Slayer.