This looks exceedingly good! Very nice map!
I don't see anything wrong with the mountains, maybe you're not found of the particular shape, or style, but they seem alright to me.

If I may nitpick, the island formations to the left seems a bit artificially shaped, like the shape you get after you use the eraser continuously. Also I think that your map lack a bit of contrast, maybe an adjustment of the histogram levels could fix that.

About rivers, Wilbur will be of great help if you have a heightmap, and on either case this is a good thread about rivers.

Welcome and good mapping!