It is a bit weird how, in D&D, Vampires have a bunch of weaknesses: garlic, holy symbols, running water, etc. Few other monsters, have any such weaknesses; a few are blinded by strong light, and that's about it.
The reason that vampires have such weaknesses, of course, is because they are a part of real-world folklore. We have a lot of stories about vampires. We don't have as many stories about, say, displacer beasts.
Of course, inside a game world, this distinction disappears... and so does the reason for vampires to be unique in their pile of weaknesses. Many monsters could have such weaknesses. This idea seems to have a lot of traction. There is even an entire PC class built around the idea that monster have weaknesses that can be exploited.
The problem, of course, is that players read monster manuals. If the weaknesses are listed there, players will be aware of them whether or not their PCs are.
One possible solution to this is the Secret Weakness.
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Edit: Additional weaknesses in comments. Add your own!