It was done in Photoshop, drawn by hand. I started with the dark ocean, started drawing shapes with the marquee tool, and went from there. First, I filled with a sandy color. Then, Select->Modify->Contract of a few pixels, fill with green for plains or dark green for forests, and so forth. Mountain ranges done exactly the same way, keep contracting and adding different colors for height. (some hand correction was necessary, some of the contracts did some funny shapes) Probably not the most efficient way, but very simple and easy to control.
Yes, I thought about that fact. The fact is that the background of the world indicates large, powerful nations that are very old and rather powerful. Some of the original rivers were expanded, dammed up in some places, released in others, and so forth to change the direction of the rivers (to aid in their trade networks, farmlands, communities needing water, etc.) Hardly any of the deltas featured on the map are naturally-occurring; most of them were created after centuries of river manipulation.
This is especially true of the Heraldan Theocracy. The Heraldan Channel runs east to west from one ocean to another, and was almost entirely man-made. It was created to allow faster travel from the ocean directly to the sea of pillars by a (unknown name yet) Guild. The pillars have runes scribed on them (hundreds of thousands in the Sea of Pillars) that have secret navigational instructions that allow those who know how to read the runes to get to the fabled Azuroth holy city.
But, I will stop boring you with history of the world. I may change around how some of those rivers run, though.
Edit: Yes, the caverns beneath the continent would cause the whole world to collapse. I will either make some changes (more support structure) on the map itself or have it understood that the entire space is not empty, it's just counted as part of the cavernous areas. I didn't want to draw thousands of tiny passages on there (mainly to keep players from knowing every path through it). I will have to figure something out for that one.