I think there are some relevant points in an answer to the 'How can I link to an external resource in a community-friendly way?' discussion on Meta. It's not exactly what we're talking about, but the spirit of it applies:
In my experience, posts with links are not downvoted if all these conditions are met:
- you paraphrase the content of the linked item (possibly omitting
details or examples)
- you identify the author (yourself, MSDN, etc)
- someone could benefit from the answer without reading the linked item
- you include information to let the reader decide if clicking
the link is worthwhile
I think a properly attributed answer that is merely a block quote satisfies the someone could benefit from the answer without reading the linked item at all criteria, but doesn't meet the paraphrase the content of the linked item criteria.
In my opinion, explaining things in different ways makes it easier for more people to benefit from an answer. Sure the dictionary may seem clear enough, but explaining something from your unique perspective could help someone have an "Aha!" moment and really understand something where reading the dictionary entry might have just been an "uh, OK" moment.
You should assume that if it was easy for you to find what you're quoting, most of the folks landing on your answer will have found it too. What can you add that will make the ELL version of the text more useful than where it came from? Could you invent a better, or more interesting, or funnier example? Could you rewrite the explanation in simpler words or with shorter sentences? Could you make it more specific to the question that was asked?
If you're finding that it's difficult to add more explanation to what you've quoted, it's very likely that the question needs to be improved. Are you sure that the question is as simple as it seems? Do you have an idea of why the author asked the question? Sometimes asking for clarification can cause a question to become a lot more interesting.