A failing many rebellions share. It's easy to define what you oppose. Defining what you want - and more importantly, determining how to achieve it without becoming just as bad as the people you cast down is much, much harder.

[What would have become of Westeros, if anyone involved in Robert's Rebellion had considered who might be suited to rule once the Mad King was thrown down? He has absolutely no idea, but things certainly couldn't have turned out any worse.]