I see you've already created a proposal on Area 51, but I wanted to point out that we already have several sites that will (or should at least; AFAIK) cover part of what you're talking about.
From the FAQ:
User Experience - Stack Exchange is for User Experience Designers, Information Architects, and Human Computer Interaction researchers.
This should cover some interface questions (not all, make sure you read the full FAQ).
From the FAQ:
Graphic Design is for questions targeted toward 2D design professionals, students, and enthusiasts.
This includes questions about:
Graphic arts theory and history; "Why" we do what we do and not necessarily the "how" in logo design, fonts & typography, visual communication
Web design
Layout and printing
Illustration
Digital media
Examples: video, Adobe Flash
Note that it also says:
but not about..
- Web site architecture and development
Examples: HTML that doesn't include CSS, server-side issues
This should cover some graphic design questions.
As always, if you're not sure your question will be accepted, read the sites respective FAQ and ask in chat (or create a meta question).