Posting a question is like being on a journey, but stuck at a crossroads. There may be many possible roads for the traveler (that's the OP) to take. They may feel they're lost and not sure they're even on the right road. They may simply not be where they thought they were. Or maybe it's foggy and they can't even see the way forward.
A good answer
A good answer is one which provides the opening poster (OP) with at least enough information to do one of the following.
Apply the information directly and proceed accordingly.
Continue as they were, having just clarified their position.
Decide upon one of several possible directions in which to proceed.
Consider backtracking down their path so far and re-plan their route.
A great answer
A great answer is one which not only meets but exceeds expectations.
A great answer provides extra information not just about what to do at these crossroads, but what's further down the road, what considerations may have to be taken into account.
It's not just answering the question - it's like looking at the method of travel. Is the traveller on foot? Take the shorter route. Is the traveller in a car? Consider the route via the filling station because you can see the dial on their dashboard is low (which they hadn't noticed).
A fantastic answer?
Yeah - a great answer - that's cool - the OP's going to love you for it.
But a fantastic answer? A fantastic answer is one which also paints a sign with all this information - and more, and leaves it at the roadside, so that all future travelers, using slightly different modes of transport, can make use of it and see where to go and what to do when you're not around to explain it all again.
Not just any old road sign painted in a thick brush - I mean a solid weatherproof information board with maps; pointers to more information on the subject; publications that might be useful; feedback from other people who have been on the same journey.
Tell a story - tell the whole story.
That's the way we should treat users.
That's the way we should answer questions.
Here's is a rare species of Answerus Fantasticus Exemplum.