'What did you do?" would be pretty common in the context of a conversation over here across the pond. It's like Continental PT v. Brazilian PT - I know it's a bit of a rub, but if you're measuring averages, there's more Americans than Brits. I actually find the difference quite interesting and part of a natural and well established evolution that occurs in languages (eg Shakespeare may have understood Beowulf 500years after it's writing, but we need a translation, and now, 500 years later, Shakespeare is a stretch for many us requiring explanation of many outdated terms), but I understand it represents a major issue to those taking a shorter view. Johnathan McWhorter's "Our Magnificent Bastard Tongue" is a wonderful book on the subject and highly recommended.
To add: For this question we are dealing with the auxiliary verb 'do'. The grammatical rule of auxiliary 'do' + 'verb' tells us that 'do' changes according to tense whereas the 'verb' stays in its infinitive construction. For example: What do you make is present, What did you make is in the past, and to reflect this we keep 'make' and change 'do' to 'did'.
Because 'used to' is a bit tricky (I've seen it called a modal and semi modal verb based on how it behaves) I am not completely confident, but I believe in this case if we follow the rules for 'do' + 'verb' we arrive at the correct answer.
What were you doing is past continuous. So, the translation would be "o que você estava fazendo?", (a more literal translation) something that was in progress in the past. (But you can also use that for "O que você fazia?). "O que você fazia?" could also be translated as "what did you do?" Or "what did you use to do?"
According to my grammar book:
The imperfect tense can also correspond to the English past continuous ‘was/were doing’, especially in the written language, where it is preferred to the imperfect continuous tense
Eles jantavam quando a polícia chegou. They were having dinner when the police arrived.
Is your preference for "estava fazendo" because of your interpretation of the sentence or stylistic or is there (another!) thing I don't understand? :)
"O que você fazia quando o bandido entrou?" (What were you doing when the thief got into?) If you are relating two sentences that happened in the past, it's better translate as past continuous (in English)... "Eu trabalhava na GM" - "O que você fazia lá?" ("I worked at GM" - "What did you do there?") So, Duo is a bit restrict for translations
It will depend on the context. O que você fazia quando era pequeno? = what did you use to do as a child? / O que fazia fazia naquele lugar!?!?! = what were you doing in that place!?!?! Duo should accept both as we have no further context to guess which one is more correct =/
One correction... In statements (or questions) with did (negative or not) in them, then yes, "use to" is the rule (as the auxiliary verb "do" aka "did" overrules the additional verbs in placing tense). However, never used to works as well. :)
For those confused by "use[d] to do" realize that like the Portuguese, only one verb sets tense in the past in a sentence. In this case, the auxiliary verb "do/did" is the tense setter. To see this in use, try a different verb:
Q: What did you make (not, What did you made)?
A: I made soup.
Q: What did you ride (not, What did you rode)?
A: I rode the merry-go-round.
Q: What did you cook (not, What did you cooked)?
- A: I cooked carrots.