I know it's been discussed to death, but you as a vampire are much weaker than the enemies. A vampire NPC poses a serious threat. What happens when you become a vampire? Nothing worth mentioning changes and you can't walk freely in day-light in the game anymore. It's not even worth "role-playing" a vampire in Skyrim because there's no guilds, there's no quests, there's no benefit, there's no friendly vampires, there's no nothing.
I wouldn't know, personally. I have no interest in vampires, so I haven't tried becoming one. Makes it hard to have an informed opinion on their implementation.
It wouldn't surprise me, though, to hear that they are poorly implemented. It seems like a lot of aspects of the game are. As if they were trying to squeeeze so much into the game, they ran out of time to properly flesh them out. Werewolves aren't that spectacular, either. Marriage is poorly done. And so on.