Starting at the Very Beginning

[Anbairui]

Hello! I just joined this site earlier today, and I just started using GIMP a few days ago.

I don't have much to show right now. In fact, the ONLY thing I have to show is my initial ideas of what the landmass will look like. I haven't even named the place yet! Anyway, here's some information:

-First of all, this map is very basic, showing the ENTIRE world. Because of that, there is not much detail for many of the individual areas, especially on the western half (they are newer ideas).
-This world is not a sphere, but instead is flat. It is large, as well. ~15,000 km in diameter.
-In general, the northern half is dryer than the southern half, and the eastern half is cold, while the western half is hot.
-The only continent I am completely happy with so far is the one closest to the center of the map on the eastern half, as well as the islands to its northwest.

I am just looking for some feedback. Keep in mind that I have JUST started learning GIMP. I am trying a bunch of tutorials, and having trouble with each one, but once I improve, I'll adjust my map. Here are some questions:
-Does it look too jagged? I like the look of the peninsulas, but I don't want it to look sinister.
-Does anything look too rectangular?
-Other than being a flat world, does it look realistic?/Could continents like these form?
-Any other suggestions.

Thank you!