I'll admit right now that I am a one, maybe two genre guy, depending on how closely related you consider Science Fiction and Fantasy. Yes, I'll read other genres, and it's even possible I'll write other genres down the line, but this is where my heart is. Fantasy, especially.
When they say "write what you know," this is what I go to. It's true that I've never dealt with daemons, travelled strange and fantastic lands or wielded mighty magics, but if there's one genre I know inside and out, this is it. I know the rules of Fantasy instinctively, the same way I know when a Chinook* is rolling in. And the biggest rule is? There are no rules.
Ok, that's a lie. There are lots of rules, and they're different if you're talking about High Fantasy, Contemporary Fantasy or Urban Fantasy. Good and evil are more black and white (or at least you know which side a character/creature ought to be on,) magic and fantastic creatures abound, and chances are someone has a Destiny. The fun part is, even within these rules, you get to reinvent the world each time. In fact, that's pretty much the point. While there's only so much you can change about a certain "race" or "species" and still have it be recognizable, you get to take it and make it your own, with your own rules. For example, there are certain things that make a faerie a faerie or it isn't a faerie, but that can range from sweet Victorian flower fairies, through Tinkerbell right to something downright malicious like Jenny-Greenteeth. Don't like what came before? Reinvent it.
Vampire stories are notorious for having a different set of rules for every author, and while each reader has eir own preference, we can (usually) recognize that it is a vampire when you tell us so. As long as we have drinks blood + immortal/unnaturally long-lived, we'll go "yep, that's a vampire all right," even if the rest of the details get changed faster than topics in an ADD conversation (though some of us still draw the line at sparkles. I mean, seriously! He's a vampire, not a disco-ball.) The upshot of this is, in Fantasy you get to change the rules. A lot.
One thing you can't do is break your own rules. Once you've established a magic system in your universe, you have to stick with it. Your trolls turn to stone in the daylight? You can't have one suddenly take a noonday stroll. Your vampires are allergic to garlic? They probably won't be going out for Italian. Whatever else you do, you have to keep up an internal logic or the reader with think you have no idea what you're doing. Keep that in mind when you're doing your world building; consistency is key.
If you're writing a Fantasy story, how well are you sticking to your own rules? Think I'm full of crap here? Tell me why. I'll never learn otherwise.
*For those who don't live just west of the foothills of the Rockies, a Chinook is a warm wind that comes in from the Pacific Ocean, over the mountains, and is known to raise the temperature above freezing in winter. Also known to cause nasty headaches from the pressure changes.