Rivers are my personal bugbear, also. I think yours look quite good on this map--they match the hand-drawn style nicely. You might play with the blend modes or opacity to give them a more painted look.
As you add details, the identical mountains are looking more and more computer-generated to me. Take a look at Hapimesis' video tutorial:
[Award Winner] Simple Mountains [Photoshop/GIMP/Painter/etc.]
You didn't mention whether or not you're using a tablet--you'll probably need one for best results with Hap's technique. I know I found it close to impossible to accomplish with a mouse. Others might have a steadier hand than me, though.