What is a sufficient length for a stream? What is too short?
I'm never catching anything live so agree with what others have said, I'll just watch the stream at my own pace. Only thing I'd mention is the FAF Live channel has a lot of quite short streams as there's typically a restart or two due to technical issues. I understand this is because the stream is used by many different people so nobody is proficient, but working out those issue before starting would give a more polished feel.
Perhaps just a checklist provided by one of the experienced streamers to go through before beginning could help? (as Jip has suggested)
When it comes to caster style, do you prefer a fun and entertaining personality or a deeply-analytical one?
Considering FAF Live typically has multiple casters at once I think the most important thing is for them to have an entertaining dynamic. CaptainKlutz and Jip did a good job of bouncing off each other for the Settling Scores cast. A pattern I've seen in other RTS casts is to have a "colour" caster paired with an analytical one, since that leads fairly naturally to a fun dynamic (as FTX suggested).
Another note on polish would be for the casters to take opportunity of 'empty' moments to introduce themselves to the chat, since people (or I at any rate) usually will know names but not voices.
What kind of content do you want to see on FAF Live? Strictly high-level play, a mix of high-level and average-joe events, anything and everything possible (the more the merrier)
I personally prefer high-level, but think a mix would be best for the audience more broadly. Another consideration is that the players, casters and tournament directors are mostly drawn from higher level players, as they're the ones volunteering their time what they want to do is probably more important than what the audience to see. And of course as most FAF Live coverage has mostly been high level play to this point, this is also what the existing audience has self-selected for.
The FFA tournament a few months back was interesting to me, as an example of non high level 1v1 gameplay.