I think there is a sweet spot. Babylon 5 was great fun but weakened because they had to squeeze too much ongoing plot into season 4 so that too little was left in season 5.Television back in the day was much better than we get today in my opinion! Story lines which are dragged out more in the line of a serial than a series annoy the hell out of me! In Trek we got a complete story each and every week which was well written, paced and executed so that it didn't really need a second episode!
JB
That said, I rather liked the way Buffy did it, with a villain of the year, and a mixture of stand alone and ongoing story episodes.
The thing I loathe most about stand alone stories is the allegorical monocultured alien of the week when I wanted to see more Andorians, Orions, or Zaranites.
TOS could have had a few more ongoing elements if certain guest stars had been available to reappear, or if they had given Rand a proper story arc.