I think this should have happened during private beta on the site's meta, but let's go for it.

As for fields, I definitely see those:

• Algorithmic Learning
• Algorithms and Datastructures (+)
• Artificial Intelligence/Machine Learning (*)
• Compiler Theory
• Computer Engineering
• Database Theory
• Distributed Systems
• Formal Languages and Semantics (+)
• Logics (+)
• Network Theory
• (Principles of) Programming Languages
• (Formal) Specification and Verification of Systems (+)

Probably also the computer science parts of interdisciplinary fields such as:

• Bioinformatics (*)
• Robotics (*)

Please feel free to expand the list and add detailed subitems. Items marked with (*) are (partly) represented on other proposals/sites, too. Items marked with (+) are represented on cstheory.SE if on research level.

As for levels, these are my thoughts:

• Soft questions as present on many sites, e.g. discussing the field, its practices or asking for general advice.
• Academic Beginner questions, i.e. coming from undergraduate students and probably relating to course material. A policy for homework questions has to be developed; I like the model from math.SE.
• Academic Advanced questions, i.e. questions students may come up with during their studies.
• Academic Research questions, i.e. questions relating to current research and open problems.
• Interested Practitioner questions, e.g. opens a textbook and is overwhelmed.
• Practitioner Problem questions, i.e. a practitioner runs into a problem at work/his hobby that needs some CS insight.

I don't think the word "level" is quite right as they might be orthogonal, but for lack of a better word I stick to it.