I'm mostly active on IT Security and we have a separate meta. Most of my questions on IT Security meta have answers with links here. So my question is, where am I supposed to post what i.e. feature request, bug report, behavior questions?
Posting on a child meta is absolutely fine!
The Stack Exchange team monitors all of the child metas. The child metas do have a smaller reach than MSO, so posting on the child meta to get an initial gauge of support, and then if necessary, bringing that to MSO for a wider audience is fine. But there is no reason to turn a post away from a child meta. Not everyone is involved with the entire network. They don't know that there is anything beyond their meta, and they shouldn't be received poorly because of it.
Think of ITSecurity as a state. ITSecurity's meta is the state capital. Meta Stack Overflow is the nation's capital. The state functions, it just gets a little help from the nation's capital.
In keeping with network policy, an answer on child meta that contains links should not just be a list of links. The post should also summarize/explain what the links are. Information can be found in many places, and we should not be forcing users to go on a wild goose chase to find information.
I can't stand it when I see a post on a child meta and the first comment is "You should have posted this on MSO". ARGGGGGGGGG
Please consider the nature of your question:
Otherwise your meta will be full of questions that have already been asked and answered on here.
Worse still, unless someone actively maintains all those duplicate questions, they will eventually fall out of date, and cause further confusion when the information posted is wrong since the engine is still evolving.
If Stack Exchange said, "It's done. No more major changes to how the basic engine works in terms of voting, abuse, posting, commenting, flagging, etc," then I'd say fine - duplicate everything everywhere.
But since there are still significant changes in the future, having all of the relevant information go to one spot is preferable to a highly fractured and duplicated world.
Further, you are likely to get a better answer faster here if it concerns the engine itself.
Lastly, the enthusiasts that like to discuss the engine incessantly hang out here, and if you have a feature request they can likely chime in with significant support and help, or with a lot of information you might not have considered.
However, feel free to post everything on your per site meta - participating here is completely voluntary, and you will still get your answers there, and when needed they will be migrated or discussed here as well.
In other words the meta sites are a special case in terms of whether something is on topic or not. If you don't know where it belongs, just post it on your own meta and let others sort it out.
To post on a child meta site is perfectly acceptable, and nothing forbids to post there.
There are cases where to post on a specific meta site makes more sense, especially if
If, for example, you have seen somebody behaving in a particular way on an SE site, and you want to know if that behavior is generally acceptable, then you can post a question on MSO; if you want to know the opinion of users to (let) decide the actions to take, then you should post the question in that meta site of that SE site.
In general, I have never seen a question on a meta being closed as duplicate of an existing question on another meta site; if there is already an equivalent question on MSO, then you could be pointed to that question with a link.
Update: @Robert Cartaino wrote a post explaining that is perfectly fine to post a question that is relative to Stack Exchange sites in general in a child meta site, as Stack Exchange Community Coordinators check also those meta sites.