Coastlines are best done with a fractal tool because fractal is what they are . Many graphic sofwares have a possibility to create or edit a fractal curve.
There is also a plug in for GIMP here somewhere (have not the link handy) that enables to create fractal curves.

For the desert on poles. Yes the relative humidity at poles is minimum on any planet.
But on the other hand the temperatures are sub zero all the time. So while it is dry (like a desert) it is also covered with ice and snow. So it is an ice desert and not a sand desert unless there is no water (oceans) on the planet.

Of course on a fantasy map there can always be some unnatural features caused by magics.