Fantasy worldbuilding is about much more than the fantastical. To make a world real, you must consider logic. Consider it early and often. Any fantastical assumptions you make must be followed by the proper logical progressions. This is a major factor with things like magic, especially because it can be difficult to think of everything that might follow a certain development, but it is also important to the economy, politics, social relations (including those between different intelligent species, if your world has more than one), and even ecology of your fantasy world.
This applies to non-fantasy as well. Let's say you're writing a non-fantasy book (for some reason ;P) set in Virginia, and you decide to make up a town because you don't want to deal with making mistakes about real places or getting sued by anyone. If setting is going to be any non-negligible part of your story, you need to have some idea of the reason your town exists and how it works: What are major employers? Is it a college town? A farming area? Historical site? What is there to do? If you want your setting to be a city of 80,000 people, then make sure they have a reason to be there. Conversely, if your town has a population of 5,000, you may not want to give it a major shopping center or university. Make your setting make sense.
Medievalesque fantasy, my main writing area, has some special challenges. In a town, city, or other settlement: What is the main water source? People will not usually settle in a place without readily available drinking water - unless you have designed a species that doesn't need it. Where do the inhabitants get food, and what kind? Clothes? What do they do for entertainment? With whom do they trade, and for what? I recently had issues when I realized that a character owned a frying pan whose society had virtually no way to obtain metal. That had to be reworked, and my understanding of the character's society deepened as I actually decided how they would use other materials in situations when we might use metal. Indeed, it led to a convenient plot point.
Ecology, too, can be a real issue in fantasy. Many fantasy writers seem tempted to throw a whole bunch of apex predators into a world which otherwise, it seems, contains mostly the same animals we have on Earth. What, one wonders, do their dragons eat? And sphinxes, leviathans, chimeras (chimerae?), basilisks, and so on? Do any of them compete with predators we would recognize? (I once saw a program in which a dragon and a tiger fought out a territorial dispute.) How many of these creatures - and the answer is often "all of them" - eat people? Can they survive on people alone, or do they also eat livestock and large wild animals (buffalo, deer, whales, or things that in this world are top-tier predators, such as bears).
(As an interesting point, it seems that humans reached their current evolutionary stage on Earth starting as prey animals. With our intelligence and tool-making abilities, we turned the tables on our predators, many of which are now endangered. Keep in mind, if your fantasy world lasts for vast periods of time - say, several generations of long-lived dragons or elves - that humans will have had all this time to find ways to defeat, or at least deflect, their predators. They may even have magic in addition to tools, depending on your world.)
In my worldbuilding this week, I found a wonderful ally in a humble little book called The Audubon Society Nature Guide to Eastern Forests. I was, at the time, trying to get a feel for what it would be like to tramp through an oak forest; we have none in my immediate area, and my characters were walking around in one. It outlined different types of forests, explaining where each was found and why: the environmental factors, from temperature to soil content, that caused certain plants to grow, and how those plants plus weather, altitude, etc. affected the animals living there. I discovered that what I really wanted was an oak-hickory forest, and was immediately furnished with a long list of species found there.
The point I mean to make here is that ecology, like all of the various worldbuilding factors, can be done right or wrong even when no fantastical elements are involved. If you have a vast tundra with no wildlife except for the occasional snowshoe hare and large packs of wolves, then you have apparently invented wolves which feed on snow. As such, they should be little danger to travelers. And in a fantasy world, if you provide no reason why a thing should be different from our world, people assume it is not. (Think of gravity: unless you say otherwise, readers will assume that your characters walk on the ground and that dropped objects fall down.) This means that there are aspects of the world where you can make mistakes based on non-fantasy things. (This is common with horses. I have read many commentaries on the lack of realism associated with horses in some medievalesque fantasy. Do not let this happen to you.) Even in a world with magic, snakes cannot wink unless you expressly provide a reason why your world's snakes have eyelids when Earth's do not. This is a reason to research anything "real" that you plan to include and with which you are not already quite familiar - even if only very few readers, those who are experts in the area, catch an error, wouldn't you rather not have that error? Besides, sometimes you come up with fun new facts, some of which you might use. I researched foxes over the past few days because a fox figures prominently in Rabbit and Cougar. I learned that foxes wag their tails when happy and that their "happy sounds" include clucks, whines, and what sound like human screams. I even read - though I need to check this - that they lack the facial muscles to bare their teeth. How interesting is that?
(Well, a lot, if you're writing about a fox.)
Even if you don't include everything - and please, don't include everything - that you know, it's usually better to know more. For example, the fact that my oak-hickory forest even has foxes implies that it has things they can eat - and I know what they are. That the forest contains mice, rabbits, insects, and berries (and oh yes, I do have specific species) may not come up, but knowing this will keep me from being inconsistent (for example, having the character who lives in this forest see a mouse and say "whoah, what's that?").
Research and good world-building makes the writing easier and more satisfying. It gives the story a deeper and more thought-out feel and allows you to confidently and correctly use specific examples. (I now know, for instance, what undergrowth grows in oak-hickory forests.) Your readers will appreciate it. And it's loads of fun - win-win!