Welcome to the forums Valstarr!
As you're already well aware, it's an open ended question. What you 'need to know' depends on your goals, and your role.
Personally, in my role, I know mostly about HTTP / Server Side Scripting / Database design, and a smothering of SEO, but I work with other people who are more marketing orientated than myself. We outsource design. I can write HTML and
CSS but can't proficiently use CSS, and I'm rubbish at design :)
Taking the slightly archaic role title of 'webmaster' you could look down the forum index of WebmasterWorld [webmasterworld.com] and produce reasons for having 'need to know' information on each topic
. Anyone who has been working in webmaster related industries will know why each of these forums exist.
From your examples it looks like you're erring towards coding, both the server side and client side.
Aside from coding the site (which covers a very large topic area)... you're going to have to be aware of information architecture in general, which touches upon site navigation and usability. If your site is going to be in multiple languages, how would it be laid out?
There's also the consideration of hosting. Which OS to use, which web server, which scripting technologies, add-ons... the list goes on. A question of who maintains it (time and knowledge) is also a factor.
An SEO may want to tell you which country the server should be hosted in, why the layout of the site should have clean SEO friendly URI's, no session ID's, vanilla HTML hyperlinks, lots of text.
The SEO will also want a say in the content of the site, particularly page title's, headings and links. The SEO may (not) be the same person who writes the content, which can be a job on its own.
When a site is built, the visitors will need to come from somewhere. This will inevitably involve lots of online marketing, and some offline (perhaps suited to someone with those skills?). This can involve link campaigns, PPC, buying paid links, affiliate programs.. the list goes on.
Progress should be measured after taking the initial industry research (keywords, target market and the like)... so someone who is interested and knowledgeable in analytics will need to put some time in there.
Customer care/visitor support is also important, and is taught in a number of degrees as a module.
Finally, a basic knowledge of OS's is good, because your friend won't believe you're doing well in your new job if you can't fix their printer ;o) ... not good for the image.
I think, all things considered, you want to know the kind of things that get you 'excited' about work, where you're working towards something you think is of value to people, or will get you rich.
If the plan is to go it alone and attempt to carve out an income 'by making websites', I'd say that focusing on SEO and content would be the priority. There are so many good content management systems out there that have dealt with historical woes like bad URI's and the like, you can focus on providing valuable content which will be highly visible.
My 2 cents. Always a great question and always a number of answers.