Most 'classic' races are allowed, barring most anthropomorphic creatures, nekojin and things in that vein. Being that my experience with fantasy races is actually quite limited, I only have rule sets for a few that I've encountered frequently during RP (namely, the ones I feel are frequently "ruined").
One common point of contention is anthros. Anthropomorphic creatures that are pretty much just "furries" - meaning a bipedal creature that has animal features but human speech and things like that - don't exist. And although they're not normally under this umbrella, nekojin and kitsune also don't exist. Anything that just has a few animal characteristics in this vein simply doesn't exist. There really doesn't need to be a huge explanation why they don't exist, they just don't. I mean, you don't get told in real life that dragons don't exist, and commence whining about how that's stupid, do you? One planet can't have everything!
Exceptions to this are things like centaurs, minotaurs, satyr, nagas, harpies, etc.. Pretty much, Greek mythology = OK, Japanese mythology = fuck off. Yes, this is an unfair bias against a certain culture. Maybe you reasonable people shouldn't have let anime dorks completely slaughter any interest I ever could've had in Japanese stories by playing annoying, cartoonish characters.
The offspring of other races and the mentioned mythological races can also exist.
With the popularity of things like Twilight, Blade, Underworld and so on, I find myself really having to put my foot down on what constitutes a vampire. So I'm going to describe the exact traits of a creature that would be called a vampire in the sense that that's its technical classification, the textbook definition (and yes, textbooks in schools will mention vampires and other things like this as part of a biology-type class!).
Vampirism is a curse. The first vampire was probably created by some vengeful god, you know how that stuff goes, I'm not going to go into detail. Anyway, subsequent vampires were created by draining a mortal victim of their blood, then feeding them their vampire blood--you know, "The Embrace".
Because of this, a vampire can be rooted in almost any other species. While a lot of vampires were human beforehand, this doesn't mean that all of them are. i.e., if your character was an elf before the embrace, that means they're a vampiric elf, not that they're "half vampire, half elf".
Vampires need blood to survive. While they may be able to survive on animal blood, it won't nourish them as well, and they crave the blood of sentient creatures above that of lesser ones.
Vampires cannot walk in sunlight. More powerful vampires may be able to tolerate moving around during the day when it's cloudy, raining, what have you, though. Of course, this is very risky, because the weather could clear up without warning. Vampires who stand in sunlight crumble away into ash (not just "POOF!", though, it takes a few minutes). PS: Combining this with magical ability could be a fun idea. . .as in, a vampire can change the weather of a given area so they don't have to hide inside/underground all day.
As suggested by the "vampirism is a curse" thing, vampires cannot have children. This means that there's no such thing as being "part vampire". Their formerly-mortal reproductive systems no longer work.
The ability to have sex - and the desire for it - depends on bloodlines, magical abilities and things like that. Some vampires will be able to do it without much fanfare, though they'll be barren or shooting blanks, depending on gender. Some will need a great amount of blood to do it, and others will need to employ magic for it. Basically, it's possible, but it's not a universal vampire trait.
Vampires' bodies are naturally cold. This can be changed by magic, and of course by having fed recently.
Speaking of magic, vampires are extremely magic-adept. Mortals that couldn't learn or control magic before the Embrace will find it easy to perform as a vampire.
Also, vampires possess superhuman strength, speed and endurance. These qualities increase as they age, and fade when they haven't had enough blood. A starving vampire will be weak and frail.
Vampires are very vulnerable to fire.
Vampires can die from sunlight, from being starved for blood, from being set on fire, and from having their heads cut off. They will recover from other injuries, even if it involves growing limbs back. (The ol' stake in the heart just paralyzes them.) Stemming from that, vampires' bodies attempt to immortalize them as they were when they were turned. This means that, for example, if they had long hair when they were turned and cut it, it would grow back rapidly, as though regenerating an injury (usually within a day or so). If they had short hair when turned, it wouldn't grow. Things like this can probably be altered with magic, though.
The above things are all the "universal" aspects about vampires that I can think of right now. Other strengths and vulnerabilities may apply, but these are usually dependent on bloodlines (i.e., whether they have a reflection, whether they're vulnerable to silver or religious items, etc.).
Also, you're free to make your own interpretation of a vampire, so long as you don't call it a vampire. Give it a different name and we're good.
These are different from shapeshifters! Shapeshifters can change into any form; were-creatures can only change into one. Werewolves are most common, but other kinds are acceptable. These usually have in-the-middle forms. I'll admit that I'm not very knowledgeable about these things at all, so I'm kind of pulling rules out of my ass depending on what I, personally, would prefer to role play with. I'm sorry.
Anyway, as a start, I'll say that the most 'natural' forms for werecreatures are their 'full' forms. They're most comfortable as either humans or as the (often monstrous) animal forms relative to what kind of werecreature they are. Holding the middle form is exhausting; the middle form results from a conscious effort not to fully shift into the animal form, and isn't likely to be held long.
Werecreatures can force the shift if they want to (as in, turn into the animal form during the day), but this usually requires a lot of effort, though this effort lessens as they become more disciplined.
Werecreatures feel an urge to shift at night, and that urge changes with moon phases. During a full moon, the urge will be so strong that they'll shift involuntarily without a massive effort against it. Only during a new moon does being human at night feel "right"; as the moon waxes, they feel more and more that they ought to spend their nights in the animal form.
I'm going to say that all werewolves are vulnerable to silver. I don't know any prevalent mythology about other werecreatures, though, so I'm not going to say that other kinds share that vulnerability.
Werecreatures' human forms have keener senses of smell, taste and sight than normal humans. They'll often have better strength and speed as well, but this difference is not as extreme as it is with, say, a vampire.
Werecreatures are so long-lived that they're nearly immortal. They age very, very, very slowly.
Whether werecreatures feed on sentient life or not is up to you. They might be raised to believe humans, elves, etc., are lower than them and they're OK to eat. They also might be raised to believe that they're equals and they shouldn't be eaten. The meat of higher creatures is probably more satisfying than eating wildlife, but a werewolf can be pretty healthy and strong just by eating a lot.
The only creation method for werecreatures that I consider invalid is being bitten, because that would result in WAY too many of them. Any other one is fine, I really don't care. I don't play with many of them.