I've only done a few regional maps and that was many years ago, so you can take this with a large grain of salt...
It looks like a good start.
But your river systems... They seen to take excessively long and indirect routes to the coast.
Of course, the terrain in the 'flat' areas might dictate those paths.
Showing hilly regions that force the rivers to flow the way they do would make them much more plausible.
Or reroute your rivers to give them shorter paths to the coast.
Oh, and that river in the upper right that flows from the mountains to the ocean through a range of hills -
there isn't an obvious cut/break in the hills where it flows through,
but there is a more logical path further south through a break in the hills.