Hey there Arx, welcome to the Guild? The land shapes look pretty good but one thing that immediately stands out on the first map is that the land outline appears really pixelated. What resolution are you working with? If the resolution is too low the map will appear pixelated. Also, you might want to check out OldGuy's tutorial on Creating Realistic Coastlines, as it will walk you through how to fix the coastlines (once you've resized the map to a higher resolution).

Cheers,
-Arsheesh