It's interesting....I've been on a Buffy the Vampire Slayer kick, having purchased some of the dvds (finally). And I'm remembering why I used to sing this show's praises until I nearly lost my voice....
There's no doubt who the star/main character of Buffy the Vampire Slayer is. She'd the lead. She's the one that brought the Scooby Gang together, the one that the villains are after....the show revolves around her. To use mscbryk's analogy: the others are satellites to Buffy's sun. Almost everything that happens to the characters (development- good or bad) goes back to Buffy existing in their world. They change, grow, and contribute. They each have their own individual backgrounds and storylines, but they orbit seamlessly around Buffy while bringing their own strengths and weaknesses to the table (the show). The show is an example of an ensemble used very well, while still obviously having a star.
In Dark Shadows, most of the characters are still. That's the difference. They don't change with the experiences Barnabas brings upon them. And their interaction with him is limited to surface scratching because they're not allowed to develop. For example: Vicki stayed the clueless ingenue until she left. Compare that with Willow, who went from a wallflower to a powerful witch. There were a few good instances of development for other characters on DS - Quentin and Willie immediately come to mind. Unfortunately, though, they weren't really utilized afterward. They were just 'there' like everyone else that wasn't Julia.
Imagine BTVS with just Buffy and Giles solving issue after issue on the Hellmouth - everyone else still there, but not actively participating in the plots unless they're in a clueless victim role. That's my problem with the Barnabas/Julia dynamic.