A Theme is essentially a way to change the look and feel of the page while generally keeping the same content available amongst all themes. It's a way to "style" the website and choose which you wish to view it in, based on your personal preference.
There are many reasons I chose to use themes. The first is, I wanted the content to be more adaptable. The "Legion of Pain" theme was very "static" and didn't offer much to those who have the capability of large screen resolutions. WotLK theme was the first of it's kind, and is intended to cater to sizes from 1024 x 768 and stretch to fit everything above that resolution. Other themes may not do so, but that's the beauty of choice, which is why I believe a themed approach to a website is a good way to go.
I also wanted to integrate the main site more with the forums, making the same links available even while trolling. The WotLK theme uses an iframe to place what once was outside content into a layer inside the theme. Not all themes use an iframe to incorporate the forums or the gallery, but the ability to do so or not to do so is always a good option to have.
YES! You definitely will be able to. I intend to put out a starter kit with the template structure and a list of functions you can use to load the different modules, etc.
Though I tried to make the core structure fairly simple, it IS of course CSS & some very basic PHP. The amount of PHP knowledge necessary to create a theme is minimal, in fact any PHP I used in the WotLK theme template was simply for convenience/shortcuts. and you should most definitely find answers to any questions you have through php.net or by asking me. I would however recommend you know HTML & CSS and a Photoshop-type program very well before attempting to create your own theme. All submitted themes will be subjected to code and image review and only upon passing said review will they be added to the list of available themes for members to use.
Well now it wouldn't be much of a "theme" if it had to, now would it. No, you do not need to make it look anything like any other theme. The entire page CSS and Template are theme based, so you can literally move any item anywhere you want, change font sizes & colors, the whole sha-bang.
However there are a few core functions you'll need to use to call certain modules, like menu(). Again, design wise you can place it anywhere you want and make it look like whatever you want, but things like menu() must exist somewhere in your theme. I'll update this list of needed items as I find them.