The main problem is visualization. You have to tweak your writing somewhat to convey what you're imagining. Whilst it's easy to describe things in the real world, it's much tougher trying to explain some monster you've just made up! You need to pay attention to this, so that your reader doesn't get too lost. Take things slow and don't overburden your audience too soon. I guess this might work from a writing perspective too. Start small and then work up. Create a city, write about that, then move up to a country, a continent, then the entire world perhaps. That's one way to do it, to ensure you're not trying to do too much at once. That's why I advocate writing a few short stories in your new setting before moving on to a longer piece, so that you and your audience can get to grips with your new world at a manageable pace.
There are a few websites I stumbled across in my time, although thinking back, not too many were very useful. I got much more inspiration from reading/watching/playing other fantasy stuff. Drawing pictures and maps is always a bonus too, to help you convey your setting.
I'm playing about with world-building at the moment, with a potential new fantasy project. Not sure where I want to go with it just yet, but I'm more than happy to discuss the process.