Nice start! I think you could get some more contrast in there. I'd start by putting a 1px black stroke around the continents, to better separate them from the oceans. A second stroke around the continents, say 4-5px in a blue a little lighter than the ocean color, will give the impression of shallows.
Something else you might try is to use the burn and dodge tools on an overlay layer to add some contrast to your mountain ranges, and maybe try to get some ridgelines in there. To do this, create a new layer above your continents and fill it with 50% gray. Set the blending mode to Overlay. You should see the new layer vanish. If you use the burn and dodge tools on the invisible layer, though, you'll see everything underneath it change in value a little. Dodge the dark sides of the mountains (looks like the southwest side to me) and burn in the light sides a bit, and you should see them start to look a bit more pointy.
I had a little bit of fun with a paintover on your image. I did all that I said above (screwing up the shallow water stroke, so that's ugly), and I also made another overlay layer, magic wand selected the oceans, and ran the difference clouds on the new layer a couple of times to get some variation in ocean depth. I used the burn and dodge tools to carve out a couple of trenches and restore the continental shelves.
It's a quick-and-dirty job; spending a bit more time would make it better, but hopefully you can see how pushing the contrast makes for a more visually compelling map. I'm looking forward to seeing where you take it, assuming you intend to keep working at it. Some more biomes would be good to see. I like what you did with the frozen southern continent. A touch of that would probably work well on those extreme northern islands, too.