I've read many partial and full manuscripts by authors who have an interest in a particular subject--be it a profession, a hobby, a locality, or something else--and it really shows in the manuscript. There are no odd inconsistencies like Jenn wrote about, the stories tend to be a bit richer because of the all the detail the authors know about their subject matter, and the author's enthusiasm for their subject clearly comes across and often sparks the same enthusiasm in their readers.
A lot of these fantastic stories are based on first-hand experience--I've noticed a lot of lawyers writing legal dramas and crime thrillers, for instance--but many of them are based entirely on research. There's no rule that says that an author who grows up in the city and has never seen a horse in her life can't write a book about cowboys--but she is probably going to have to do extra research to be able to do it convincingly. And, if she does do enough research and really puts effort into it, she may be able to write a cowboy story that is even better than an author who grew up on a farm and made the Olympic equestrian team.
And then there are the authors who neither have first hand experience nor do any research at all. And the interns cry.
This problem most frequently arises in books that are based around a certain technology, especially the internet or cell phones. Remember that knowing how to use a device is not the same thing as understanding how it works. For example, many people own hair dryers. You don't need any research to tell your reader that your character dries her hair with a hair dryer--even people who don't use hair dryers have a basic understanding how they work (plug it in, turn it on, and hot air comes out). But say you have a MacGyver-esque protagonist who opens the hair dryer, crosses a few wires, and, magic!, the hair dryer is now a car. We know this can't actually happen, and it's so ridiculous that the reader won't be able to believe your story and will probably stop reading (unless your story happens to be a parody of MacGyver).
The key is to keep in mind who your audience is and how discerning they will be. Writing a children's book in which monkeys' fur turns green when they eat a lime popsicle is okay because it's a kid's book and they don't care if that can't actually happen. But if you write a cowboy book and you have no idea how to ride or care for a horse, it's time to go down to the library or stables and learn because many of your readers are going to be cowboy fans and are going to think it a bit odd when your protagonist reveals that his horse only eats chocolate cake. Likewise, if you write a regency romance, you better not have people driving around in SUVs.
Again, this isn't to say that you should only write about things you have first hand experience with. But I am saying that if you don't know the subject you're writing about, you need to learn about it because most times you don't need to be an expert to tell an author hasn't done his or her research. The more you know, the better your story tends to be.