Other skills include,
- Being a ninja.
But aside from that you pretty much summarized the core basics of what you need to familiarize yourself with to learn and develop with the WordPress platform.
- HTML
- CSS
- PHP
- jQuery/JavaScript
Are what make up the foundation of WordPress itself. How far you want to go with each of those is up to you, but don’t be overwhelmed because despite there being some complexity involved with learning to develop, and develop well, that learning curve will eventually tip in your favor if you persist at honing your skills in the aforementioned.
The first place to start is with the WordPress Codex which can be found here http://codex.wordpress.org.
The Codex is the official documentation for the WordPress API system and much more, literally covering every aspect of the platform, development and its powerful list of functions. You will be referring to that a lot and you can learn just as much from the Codex by studying it and the example code provided within.
Fortunately there’s no shortage of WordPress related resources around the Internet, without a doubt WordPress is the largest, most supported platform and a result tutorials, snippets and help concerning all things WordPress design and development are not in short supply.
I recommend sticking around this site too as its an invaluable resource for technical, code related questions for WordPress development and you’ll no doubt make use of its vast wealth of knowledge contained within the question and answer threads.
There’s many other resources I could recommend and hopefully some other people throw a few suggestions up too but in the meantime, if you haven’t already, check out WP Tuts+ http://wp.tutsplus.com/ – which is one site that has a lot of great WP related content.
Nothing will beat getting your hands dirty by jumping into some theme building, even if just the basics like Pippin said.