I think the first question to ask is... how much magic is involved? This is critical as the amount of magic involved will determine how things take shape. I think you're going to need some immediate answers to the following questions:

1. Is there a magical force that is keeping the world flat? If not, then over time, it's going to deform itself into a spherical shape due to gravity.

2. Are there two sides to flat world? In other words, do both sides of the planet have an atmosphere - making it, in a sense - kinda like the two separate faces of a coin?

3. How does the atmosphere work on flat world? Due to the awesome link from Tilt we know that if left to its own devices, the atmosphere will most likely form a "bubble" around the center of flat world due to gravity. This means that if you began walking the atmosphere would get thinner and thinner, and you'd eventually need a space suit to survive.

4. How do continents and mountains form? They normally form as a result of plate tectonics, but if those don't exist on flat world; then eventually wind is going to erode your mountains away and the land itself will be left pretty flat.

5. Does flat world have an ocean, or is it almost entirely land with inland lakes and rivers? (If flat world is mostly land, then it would eventually form landlocked seas where all the water would drain, and these seas would likely be at the direct center.)

6. What is flat worlds relationship with the sun? Does it have an axial tilt like earth, and does it rotate... and if it rotates what does that rotation look like? This will have a huge impact upon what weather will be like on flat world.

As you can see... the biggest question here is: How much magic is involved? At the least, you're going to require magic that keeps the planet flat, otherwise, as I said, it's going to deform into a sphere over time.