I guess theat there are just some general patterns in television.
For example I've never watched a show where the second season wasn't a hell of a lot better than the first (which is why I don't understand networks who can a series after an entire season- either do it early on, or not at all until the end of the second).
Then, if a series gets a second, they can build the characters, plots and mythology into a three series if the writers are any good. The third season either slightly better or slightly worse than the second.
Then by the end of the third series a lot of the original creativity has usually been played out, so you either do something a little different (like send them all to college for example) or bring in fresh writing talent, or both.
But if a series makes it past 4 then I tend to think their on safe ground writing-wise, they have established plots/mythology, well-rounded characters and so forth.
Note that this pattern that I've seen only really applies to genre shows and dramas, not procedural shows (like Law & Order, CSI, NCIS) which in my mind are almost completely interchangable season-wise.
Even after 4 series the characters are still wafer thin, you're pretty much watching it for the plot.
This pattern also doesn't apply for ST: Voyager and Enterprise, which both sucked from the first. 🙂