Worldbuilding is *really* important.
If you're writing high fantasy or something in that vein, then solid worldbuilding is an absolute necessity. This is where you establish the rules of your world. Even if "that's the way it is" you still need to offer some solid explanation as to why people walk upside down or are blue, or at least make it so believable that I can buy it without having to suspend my disbelief.
Writing urban fantasy or Steampunk (set in our world), or dystopian or the like is, in my opinion, harder than high fantasy. When something is set in our world, you can't really play the "that's the way it is" card. The differences between our world and the one you're creating need to be explained. Not only do they need to be explained, but they need to be explained in a way that's believable and logical. You may even have to use science.
Yes, yes, I know these are fiction stories, and in a way they're fantasy. But because they are set in some alternate version of our world everything still needs to make sense, and not make me have to suspend my disbelief, because when you do that, you pull me the reader out of the story just a little bit. Where readers may be willing to suspend their disbelief a little, forcing them to do it over and over may cause them to put the story down, or roll their eyes, or what have you.
Also, teen readers seem to be a little less willing to suspend their disbelieve as much and as long as adult ones, so keep that in mind.
So, how do you build a solid world?
Well, if you're setting it in some alternate version of our world then you need to know how our world works first before you start ripping it apart. (Same goes for Steampunk -- know how Victorian London or 19th Century China, or Wild West AZ works backwards and forwards before making it your own).
This is very important because you have to know the rules before you break them and this research will show in your world building.
Be ready and willing to explain difference and make sure it's logical both to your world and to the human way of thinking. You may need to use science (and if you do, do your research.)
If you find holes big enough to march pink elephants through fix them and fix them in a way that makes sense and isn't a quick easy fix (because the readers will catch that). You may have to re-write things, and yes, it's painful, but believe me, your story will be all the better for it.
You need to know how every little thing works in your world. Yes, everything.
But you don't need to tell the reader.
How do you know if you have enough (or too much)?
This is where savvy beta readers are worth their weigh in chocolate cupcakes. Ask them to tell you what's believable and what's not, where there are holes, what they don't understand, when you have too much or not enough info, and such. It's better to have them asking the hard questions now when you can still fix things.
So have fun, but remember that your world needs to still make sense or at least have an explanation that the reader can believe.
What's your favorite part of world building?