What's it like to live for hundreds or thousands of years?
When we create elves or vampires or ancient demons, we assume their life and their life cycle must be something like ours, and that the mind of a near-immortal must be somewhat like our own. But think of how much you have changed in the last ten years. Think of the person you were then, and the person you are now, and imagine seeing that much internal mental change every ten years over centuries. We don't stop evolving. Ever. We have not stopped evolving as people, let alone as a species. We stop changing, learning, growing, only because we die. If we didn't, what would we become?
Creating something inhuman is difficult for us. Often, these creatures, species, characters become an extension of humanity. They're just like us, only their culture is different. But would it really be that simple? Our culture changes as quickly as we do. Most of us have trouble imagining what it would be like to really have lived just a few hundred years ago. Our "ancient" cultures are only several millennia old, and that would be mere generations for an elf or a vampire or a demon (assuming they don't live even longer).
Often we think that this would slow them down, that their rate of change would slow with their span of life, and therefore it would balance out and they would be... pretty much just like humans, only more traditionally oriented.
But that fails to take into account the vast range of humanity. In the US, we change like its going out of style, but there are other countries, other cultures. Cultures in which tradition and history are valued and in which some times of change are slow, but others aren't. We're not making elves, we're making other humans. So what would it really be like to be a creature that is so long-lived?
We can never really know. We can only imagine, but so often we fail to really take the diversity and potential of a species into consideration. Elves become a single monolithic society because we think that long-lived means resistant to change. But if elves live in a world that is changing, whether quickly or slowly, wouldn't they adapt eventually? They've been around a long time, or have they? Has that first generation even died out yet? Do they know what death is like for them? Has any of them ever even seen a "natural" death among their people?
That would be terrifying. To know that other species die, and yet to not know if you do. You may assume immortality, but if you can be injured and killed, you have to know that a "natural" death is a possibility. If your body can fail, surely it's only a matter of time until it does?
The list of questions for a long-lived species changes. You go from wondering which characters know their ancient history to wondering which characters lived it. How do they perceive their own beginnings? When did they begin recording their history? How do they perceive the beginning of their evolution? Do they remember the moment in which sentience began?
For instance, the human species (this is speculation of course, but from the evidence we have) only reached behavioral modernity around 50,000 years ago. There's a lot that had to happen after that, most of it is lost to us. But if the elven species had developed around that time, and each generation lived approximately 5,000 years, that would mean that they're only ten generations old. How much mutation can you get in just ten generations? Mutation is what produces variation within a species. We adapt to environments because we selectively choose mates which are adapted to the environment (which we know because they're still alive and more healthy than those without those adaptations) and then our children have those adaptations and so live longer and have the capacity to have more children, who... etc..
So, elves would possess the same drive, to choose a healthy mate, and therefore would certainly be capable of adaptation. Or would they? Maybe they don't, but because every living thing on the planet does (or is theorized to) why wouldn't they? I'm willing to accept they might not, but I'd want to hear why they wouldn't, you know? Is their reproduction different than ours? Are they themselves a genetic mutation lacking that drive?
The point is that if elves (or any natural, long-lived creature) have only lived in your secondary world for as long as humans, you might consider their evolution when you're considering who and what they are. If they were here before humans, that gives them a fairly good view of where we came from and how. Plus, how would human evolution have changed with the addition of another sentient species?
Or, consider vampires, they're made of humans, right? If they are in your world, than they couldn't have existed before humans did, (unless they were made from something else then?) but their age is going to play a big part in who they are. Truly old vampires have had a long time to grow, evolve and change and they would do so by the definitions of their world, their reality, in which there is nearly infinite time. (Infinite! They could, conceivably, witness the end of the universe, if they can get off this rockball before it gets smashed and maintain a food source, of course.) If that reality contained humans (meaning, if they associated with the humans it has to have at least peripherally contained) they may still retain human characteristics (ideals of morality, behavior, etc.). But they have to think about what they're going to be doing next millennia. That's like, the next ten years for them. Do you think about what you'll be doing in the next ten years? Did you think about it more the older you got? (Now, possibly, that's because you and I know we only get so many years, but the point holds if only because a vampire could live to see the next millennium.)
Some people do, some people don't, but my point is that, when you're immortal, you don't have to be static. (Or perhaps your view is that immortality leads to status, and therefore vampires, et al., are unchanging. Great! I'd love to read a story that thought about it. The point is to consider.) I don't think the idea of scale, life cycle, and timeframe are often considered when vampires, and other long-lived species, come into play. We see things in a very human way (for, I would hope, obvious reasons), but stepping outside that view is worth the time and occasional mental gymnastics.