Possible spoilers below.
The Leviathans plot had more than its share of creative gaffes, but on the whole, I hold it in higher regard than most. For one thing, we weren't able to get DS where I lived until 1969, and when I was finally able to start watching the show regularly, the Leviathans episodes were just beginning. Having only seen DS sporadically up to that point, with no real sense of its continuity (such as it was), as far as I was concerned, the storyline was business as usual.
Personally I was thrilled to see Barnabas as a bad guy, and to this day, I still prefer to see his wicked side rear its ugly head. When he's just an out and out good guy, he's just this side of impotent, figuratively speaking. He's too powerful a character to be reduced to such infuriating indecisiveness and/or dependence on Julia. His presence as a formidable force seemed to decrease over the run of the show, as more and more we see him wringing his hands and wondering what the hell to do. I'm not saying he should be a total monster, as during his debut; but when he shows cunning, quick-wittedness, and the ability to act--even if it's evil or amoral--he's all that much more "heroic." Or anti-heroic, if one prefers.
There was a distinctly eerie quality about the mystery of the noisy thing in the antique shop. As a kid, the heavy breathing behind the door scared the hell out of me; even now, unknown noises are one of the things that can still give me a case of the creeps. At age ten, I was unaware of the (marginal) connection to Lovecraft, but the Leviathans did manage to touch some of the same nerves that Lovecraft did in later years. The emphasis on a distinctly supernatural threat worked well for me then and still does.
I've got a soft spot for evil zombies, too. So when Jeb summoned up a bunch of dead guys to walk around, that really fired up my youthful sense of dread. Watching it all these years later, it hardly has a similar effect, but I still enjoy seeing what really terrified me in those days. While it may have not come off all that well dramatically, in spirit, it was just the ticket.
I have always enjoyed the return of Paul Stoddard. Dennis Patrick was so damn good, even as a kid I was moved by his relationship with Carolyn and took his death as a real tragedy. Also, Barnabas and Angelique coming together again in the way they did was a stroke of brilliance. There was some honest-to-God DS excellence in this storyline.
Sadly, the Leviathans episodes were also marred by really terrible elements, too numerous to mention. I never thought Jeb Hawkes was much of a character. No self-respecting, murderous, half-human spawn of the Great Old Ones would turn into such a wuss. That's not to denigrate Chris Pennock's acting--he's one of my favorites--but the script they gave him was usually awful. And Sky Rumson... hoo boy. I need a Pepcid.