Is it appropriate to ask questions about things like Redis (a data-structure server)?
Sure, so long as they are expert level (or at least, not beginner stuff, please).
The premise of this site is that it is for professionals -- or people who want to be professionals -- who are very experienced with databases.
Yes. No-SQL databases are encouraged here. The intent of the site is for all non-filesystem data storage analysis, more or less. If it's memcachedb or Oracle 11G, we want to talk about it here.
We only ask that you keep things objective, of course.