To me - and I found Van Helsing entertaining if not actually good - the biggest problems in the film is that the script is weak, that there are too many characters and most of them get wasted by the weak script and that there's very little sympathy for the villains. Roxburgh's Dracula is haughty and petulant, the Brides simpering. The wolfman is a non-entity. Frankenstein's monster gets the best play for being a sympathetic character but is saddled with too much silly visual gags. And speaking of silly visual gags, why does Igor keep turning on his zap stick as he falls to his death...gah that's just awful.

Tying Van Helson's mysterious origin into a huge mythic angel-demon fight was silly; resolving their conflict the way they did in the final fight was infuriating.

I like Sommer's Mummy films (and I'm biased in this as I tend to like most films with a Mummy in it) but I tend to think that the structural narrative in The Mummy is cleaner, and the romantic plot of Imhotep's brings some sympathy for the villain (which is cheated in the second film, IMO). It also tries a bit of serial cliffhanger whizbang which I think works better in an Egyptian setting than in a creeky gothic one but they also have time for quiet scenes (like Rick talking to Evie where she gets drunk and declares she's a librarian). Van Helsing's quiet scenes consist of plot exposition with swooping cameras.

Also The Mummy films also has a sense of humour that I don't think Van Helsing shows at all. The VH characters are grim and driven and ultimately ciphers. Which could be gotten away with in a movie that was heavily and well plotted, but this isn't. So it sums up to a bit of a mess.