Wordpress and other solutions didn’t just start with hundreds of themes and easy customizability. In fact the effort to customize them is not easy, it only seems easier due to the years of experience and knowledge that has been documented.
Discourse is still very new, it is still being created as we use it. Things like customization have not been totally fleshed out, but we are in a great position to be able to contribute to that process early on.
Given some more time, we will have easier customization options, and a way for those with more knowledge and skill to make themes and changes that those of us that just want to install something can.
We should learn from previous projects like Wordpress, but not blindly emulate them just because they seem successful. So I enjoy the Discourse teams caution in some areas, because a hasty badly thought out feature to appease “customers” can hurt Discourse in the long run.
Edit: I wanted to add that of all the forums software I have used, Discourse is the easiest to customize yourself. What it is lacking currently is the marketplace-like features, or ease of installing someone else’s themes. Though through plugins, that is pretty straight forward but again not as easy. Having heavily customized vBulletin over the years, being able to use modern development practices is a dream. I gave up updating my forum when it was still vBulletin, now I actually look forward to finding new things to enhance on my forum since it is running Discourse.