This is all so hypothetical. Let's make it clear:
- You can't tell people not to vote up something they see.
- You can't tell people what to vote for (i.e. the "usefulness", the "lack of context")
Thus, if you want to stop bad or poor posts from being created in the first place, you need a stronger reinforcement.
What kind of reinforcement do we have?
- Commenting
- Downvoting
- Flagging as "not an answer" or "low quality"
What if an answer is bad, but (partly) useful?
If an answer is bad (i.e. short, lacking context, lacking background), but probably correct or partly useful, it won't ever receive a downvote in our system, because it's not "not useful" per se (as indicated by the downvote arrow). Downvote it, leave a comment, and you'll be soon experiencing the rage of somebody who just wanted to help and doesn't understand why they got a downvote.
Thus: If you want people to write better answers, be obnoxious unpleasant and tell them.
It would be great to expand your answer – it is currently lacking context [add other points here]. Please look at our short How to Answer guide to see how you can make your answer better.
This means telling them repeatedly. This also means not immediately making friends, but that's not what we're here for in the first place. Super User is not a social network.
I haven't seen these types of comment very often and it'd probably be about time to make use of them.
We should particularly watch new users who aren't used to the concept of our site and gently remind them how they should write answers (example here and here). Edit their posts, show them what it could look like and leave a positive example.
What if an answer is bad and not useful?
This is such a big problem. We hardly see downvotes on answers ever. Why is that?
Users don't want to lose reputation because of someone else's fault.
So I see two ways:
- We flag those answers (but they'd have to be really bad)
- We make downvotes on answers free (just like we did with questions, where I saw a positive outcome from the change)
- We try to care less about our reputation and downvote more often.
And this is where I'm stuck. I don't know if "free downvotes on answers" has been proposed already, but if we want to tackle this problem, this is where I suppose we should start to rethink. If (long-time) users were more "afraid" of downvotes, they'd write better answers from the start.