I've tried a couple RTS games before, but didn't last long. I did ok in Dragon Commander on easy a few betas ago, though (the last few betas continued to improve the AI, but I didn't play enough to improve my own skills; I have not tried the just added casual difficulty yet).
There is a significant single player campaign, with a story line for each of the 4 princesses and 4 generals, in addition to the main plot. There are 5 advisers, each with their own story arc if you gain enough favour with that particular race. There are more choices and consequences in Dragon Commander than in any of the previous Divinity RPGs.
Multi-player drops the story and RPG aspect on board the Raven (the command ship) and just has the turn based strategy map and RTS combat. In the campaign you can auto-resolve battles, and in single player pay generals to lead your troops, giving a boost to your chances of winning (which vary depending on the type of battle and the general).
There are a few press previews of the single player campaign (the pre-order beta is currently just multi-player, but the press beta contains the first part of the single player campaign). The PCGMedia preview
focuses on the RPG elements.
Larian just did a playthrough of the single player campaign into chapter 2 on twitch.tv
, but skipped though all the story stuff to avoid spoilers.