Complete Monster: Jacques stands out as a stark contrast with his two Woobie, Destroyer of Worlds co-conspirators. While his partners had no idea that praying to their cat god for help in their pursuit of vengeance would lead to their transformation into monsters, Jacques was a normal man who willingly became a monster to make himself immortal. For years he's been using his ferry to escort people to Moonscar Island, only for him and his partners to drain the life from them. Afterwards their victims are left to wander the island as zombies, aware but unable to move on to the afterlife.
Evil Is Sexy: Lena and Simone, before their transformations into cat creatures.
First Installment Wins: Oodles of direct-to-video Scooby-Doo films followed the success of this one, but few if any of them earned as much acclaim.
Fridge Brilliance: During "It's Terror Time Again", Scooby has fallen into quicksand, and Shaggy is looking for a vine to pull him out with. One of the zombies taps him on the shoulder and gives him one. Shaggy nods in thanks, realizes who gave it to him, and skedaddles in terror. It looks exactly like one of the stock gags that Scooby Doo has used for decades. However, since the zombies are the good guys and genuinely are trying to help, this time the implausible behavior is fully justified.
Lena claims that the tunnel under the mansion was dug "to hide from Union soldiers." We later learn this is not the case. But even before this, there's a few clues suggesting she's lying, if the island's history is any indication. It's so remote that it's difficult to get to even in modern times and is in a bayou that few people enter for any reason, so the chances of the islanders being visited by the Union Army with any regularity are slim. Note, too, there are fewer Union soldier zombies than Confederate ones, further supporting the notion that Northern troops rarely came there. From this, it can be gleaned that the tunnel was not dug for the purpose Lena claims.
Why does the ferry only take people across Moonscar Island and back during the daytime? Because during the night many zombies show up and attempt to warn them that Moonscar Island is dangerous. Inevitably they will heed the warnings and attempt to leave the island, depriving Lena, Simone, and Jacques of victims. By stranding them on the island, the problem's solved.
If you watch closely, you'll notice that the Confederate army zombies only ever seem to come into view when Shaggy and Scooby were involved. From their view of the ghost in the mirror, to the chase in the swamp where they helped Shaggy save Scooby from quicksand, to when they threw themselves at Lena and Simone to save Scooby and Shaggy, and even in the last time we see any of the ghosts when the lead Confederate salutes Scooby while thanking him before vanishing. If you watched the Boo Brothers movie, you know that Shaggy's great or great great uncle Beauregard was a Confederate Colonel. Given that any time these zombies showed up, they were trying to help or warn Shaggy and Scooby in some way, it may be that they either believed that Shaggy was his great/great great uncle, who was a colonel that they knew, or they recognized that he was a descendant of said colonel and were trying to help save him and Scooby by extension for that reason alone.
It's unlikely that the gang are the only ones the zombies appeared to. Unlike the gang, who'd seen their fair share of Scooby Doo Hoaxes and became cynical of real monsters, the previous visitors wouldn't dismiss these zombies as fakes and realize they're real. Imagine being a tourist and going to an bayou island for a nice vacation, everything seems fine, the hosts are lovely and friendly, then ZOMBIES ATTACK and you're left defenseless as the undead keep coming after you. You want to leave, but you cant't. You can only hope the ferry comes back before you die. Then the ferry comes... and the captain turns out to a cat monster who wants to drain you of your life force until you're a shriveled husk. Since the hosts were so nice to you, you decide to warn them that Jacques is secretly an evil cat monster. But as it turns out, they're also cat monsters who proceed to drain you alive. Even in death, you can't find peace and your only recourse is to join the zombies and try to warn the next batch of visitors about the women and the ferry driver and hope they don't make the same mistake as you did.
Les Yay: Simone and Lena carry this vibe when you stop and realize how old they are and how much of their lives they've spent together. Also doubles as Opposites Attract considering their different personalities. Lena may be Ambiguously Bi considering the way she flirts with Fred and mentions she "really did like him" before trying to kill him.
Also, Lena wasn't around when Velma started floating, and someone had to have moved her voodoo doll. And Velma's skirt happens to be lifted up.
Misattributed Song: "It's Terror Time Again" by Skycycle doesn't have a full-length version, not even for the end-credits. A full-length mostly-synthesized version is on the soundtrack album for Scooby-Doo! and the Witch's Ghost, attributed to Gary Falcone and Joe Pizzuco.
Moe: Well, ain't Lena just the cutest little Southern chick you ever did see? It may or may not be an act; we don't see enough of her pre-curse to judge what her personality was like back then, but she definitely uses it to fool people.