John Karlen's portrayal of Willie Loomis, IMO, is the real standout for me in the original show. The problem with all the reboots is that they keep trying to make Willie more of an uglified Renfield caricature (while also trying to instead romanticize and prettify Barnabas more--the original was not exactly "pretty"), whereas Karlen's portrayal couldn't be further from that. He was closer to a Rebel Without a Cause (complete with the jacket) type. Karlen was one of the strongest actors on the show (clearly one of the solid character actors who wasn't limited to the acting range of the typical soap actor) and a very underrated one.
Katherine Leigh Scott's Maggie, who basically got all the juicy material that every version since has merged into the blander (but initially the lead in the show's introduction of her taking the train to Collinsport and seeing it through her discoveries) Victoria character (who Barnabas was also eying as a Josette reincarnation), also tended to be one of the best performers.
Willie came into the story as an accomplice in Jason McGuire's thievery plot (which is how he finds Barnabas when he goes looking for the bejeweled ring in Barnabas' portrait in his coffin), gets his comeuppance in the worst way by becoming a captive abuse victim of Barnabas (infamously shown getting caned) when the vampire was full-on villainously evil. Willie then keeps trying to protect Maggie (subtly in love with her), despite being in charge of holding her captive when she's being hypnotized into becoming Josette's reincarnation by Barnabas, and ultimately helps her escape captivity. Sadly, he and Maggie both got royally screwed in the antiheroification of Barnabas. He tries to warn Maggie about Barnabas, only to get shot and sent to a nuthouse. Maggie and Willie both end up with really annoying amnesia arcs to essentially give Barnabas a free pass for when he was a villain (while Julia moons over him unrequited while Barnabas is just fixated on recreating his long-lost Josette). House of Dark Shadows essentially plays out the original plan for the series before Barnabas got too popular to write off (it also kills off most of the cast, alternate reality-style). Barnabas was originally supposed to be a short-term villain that would die (he obviously took over the show).
Despite my frustrations with the follow-up to what Barnabas did to Willie and Maggie (it's easily my favorite arc), it's really a significant milestone in how vampire fiction developed into the types of stories we see in the genre today. Barnabas went from being pretty horrifically evil to being a prototype for the antihero vampires we see constantly today. The vampire cure arc also is a milestone in how we started seeing antihero vampires trying to regain their humanity. And then of course that's followed by a big arc of backstory with witch Angelique cursing him and the whole Regency-era backstory with Josette.
If the reboot does anything, I dare them to be more faithful to Karlen's James Dean-esque Willie than the throwback traditional Renfield reboots since, to actually introduce Barnabas as a villain initially again (it would immediately separate it from more recent vampire shows), but then to not chicken out with the consequences of his actions when he begins to become an antihero by fridging Willie and Maggie into amnesia/asylum plotlines. There's room for better followthrough with the consequences of Barnabas' most famous story (his introduction) and to make the turn towards antiheroism feel more earned.