Van Helsing is a dumb movie
The characters are all stock. It's as if the writers were checking off boxes on a list. Love interest? Check. Comedy relief? Check...
There is too much unlikely exposition in the form of scenes where one character explains to another character something he already knows, just so the audience gets a chance to hear it.
The Talking Villain is a problem throughout the movie. Again and again, a villain takes the time to taunt a lead character when simply killing him or her would be much simpler, faster, and more productive. The fact that the female lead comments on this does not sufficiently excuse it.
Now on to more specific stuff. There are spoilers ahead.
Dracula's weakness is revealed by a painting that moves. How did this painting come to exist, and how does it do what it does? Even granting that the painting could move at all doesn't help much. There are two possibilities: it moves for everyone, or it only moved for Loyal Sidekick. If everybody who looks at the painting sees it move, you'd think the painting would have long since been sold for a huge price to some art collector OR it would have been burned out of fear of witchcraft. Probably the latter. On the other hand, maybe the painting only moved for our Loyal Sidekick. In which case, why would it do that? How would the painting know the right person was examining it? Then there's the question of the information being conveyed: whoever made the painting knew that werewolves are dracula's deadly weakness. How could this knowledge even be known, given that there's only one Dracula and he has thus far survived every attack?
Dracula was exiled to a frozen land, but was given the power of flight in order to escape it. So our heroes go to this land to kill dracula, traversing a one-way portal. How did they then escape? Were they, too, given the power of flight?
Given that Dracula knows werewolves are his weakness, why does he keep one around? Sure, a werewolf is useful, but it doesn't seem like it's useful enough to make up for that rather major drawback.
Werewolves turn into wolves when the moon is full. Our Glorious Hero gets attacked by a werewolf who is in wolf form due to it being a full moon. Then we hear that having been bitten, V.H. has only three days until the next full moon. Is the moon full twice a week in translvania?
Where did the team of horses get the magic power to jump arbitrary gaps?
How did the top of the carriage catch fire?
Did the booby-trapping of the fake carriage make logical sense?
Given that the acid surrounding the syringe is strong enough to dissolve steel bars almost instantaneously, why isn't it strong enough to dissolve the needle of the syringe as well?
Why would the harpies have functioning reproductive systems to produce dead bags of glop? (On the other hand, at least now we know why the harpies are so upset - you'd be bitchy too if you'd had to give birth a thousand times and all you had to show for it was a bunch of dead bags of glop!)
Why don't the bags of glop dry out?
After all this buildup to the effect that Dracula will be able to enlighten us as to the nature of Van Helsing's missing memories, what is actually revealed? "You're really old and used to be named Gabriel" just didn't cut it for me.
What were all those cables hanging from?
If you can answer any of these or have more items to add, please leave a comment.