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
Layout and printing
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.
This site is about (from the FAQ):
Answers.onstartups.com is a site for entrepreneurs starting and running new businesses.
Topics include financing, hiring employees, legal, marketing, sales, compensation plans, banking, payroll, benefits, and more. This is the place to come with specific questions or to seek specific advice from your peers.
I think this might cover some questions about marketing..
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).