Halloween. Yay. There is a reason I've begun dreading this holiday: New vampires. It seems like the easiest way someone can prove how much they love Halloween is by coming up with some new species of vampire. Now, don't get me wrong, I sort of understand why people love them: The angst, the immortality, the power, the not having to do a regular job. I can live with that. But it's time to try something new: Do it old school. There's a good reason the old school vampire works; the combination of super-human abilities, fangs, need for blood, fangs, and weaknesses such as sunlight make a formidable foe with a reason to hide. But, if you need to change him, here are some ways people get this cool monster wrong.
The New Vampire: Someone gets bored of the old vampires and so decides to come up with a new one. He takes away a weakness or two, adds something new, and voila: New boring monster. Or hero. Whatever. However, there is a reason that the old school vamp works: If the vampires have no weaknesses, then they have no reason to hide; the more damaging those weaknesses, the more reason they have to hide. Look at this from a strategist's viewpoint: Give a vampire access to enough blood, and a small number of them can take on an army. Better yet, the vampire only needs enough blood to get started; those he slays provide him with replacement blood. If you attack the right places, the vampires can take over the world in a few short years.
Not enough people write about vampires taking over. Give the vamps some counseling, combat training, and just tell them where to strike. For some ideas, check out Kate Locke's "God Save The Queen."
The Scientific Vampire: This is even worse. An author does his homework, and finds out how to scientifically explain some of the issues with being a vampire and explains away other details as folklore. This can work, but it requires some thought beyond wanting to debunk the supernatural; you need a desire to tell a great story. The British TV series Ultraviolet works great, but that's because they also use some the weaknesses to come up with some great weapons, and they keep what makes a vampire fun, namely that they have to deal with their longevity somehow and that humans outnumber them, so they have to keep hidden. Read: The keep the drama and keep the horror coming. Too many scientific vampires have had all of the fun drained out of them along with the mystery, and they become just part of the background. That should never happen with a monster with a vampire's pedigree.
PS: Energy vampires suck. It is an attempt at creating a bloodless vampire. Although it can work in superhero and SF comics, it usually comes off as, well, lacking a certain sanguineness. So don't do it.
The Omnipotent Vampire: It may sound like a great idea, but somewhere along the line someone comes up with a vampire that is essentially invincible. In a way, this is where Vampire: The Masquerade (the RPG) jumped the shark; once it allowed vampires to be really old and therefore powerful, there was really no reason to play anything else, and the game was no longer fun. The same applies to writing; a really powerful vampire either needs to rule the world or become a bogeyman, otherwise people are going to come after him. A writer needs a character who can be beaten, or it just becomes boring really quickly.
The Angsty Vampire: Last, and probably least. Usually combined with one of the above with the idea that it is a limiting factor, and it usually does not work that way. The theory is that girls like guys that are sad, and so want to cheer them up. Guys know this; that's why they use sadness to get chicks in bars. However, somewhere someone got the idea that a vampire should be sad about living forever, and the idea spread. Now, it works for some characters, such as when the vampire has a past he wants to work, such as Aiden in "Being Human", or Spike, but that's because they recognize the issues with being a vampire and make allowances for it. They have accepted that they can be murderers and they have made their peace with it. Edward is still hanging high schools banging high school girls, knowing that he either turns them, creating a problem with the local vampires, or they die. I personally believe that if your vampires are acting like humans after fifty or so years, other vampires are likely to kill them off. So just stop it...