What is desired from our translations?
Considering that this is intended for real-world use, it seems to me that the ideal translations would use English idiom as well as conveying the correct meaning. This sometimes requires using different phrasing (for example, changing the passive to the active voice), or different words that carry a more precise meaning in the English translation. However, we seem to be graded on how closely our English sentence adheres to the original, which is closer to what's expected in a language class. It would be very helpful to have some guidance on this from the good people who are making use of our translations. :-)
Thank you @JoeyH and @andrewduo! I have written about this before (http://duolingo.com/#/comment/63381), and absolutely agree with you. I wish Duolingo would provide actual translation guidelines so we were all on the same page, but, so far at least, they don't seem interested in that. Until they do, everyone is on their own to decide what constitutes an appropriate translation and what does not. Unfortunately, many, many translators seem to be from the "literal word-for-word" camp. Unfortunately, they also penalize less-than-literal translations when ranking.
In my own opinion, we need to stay literal when we can, but the translated result must sound natural in English. That often means substituting English idioms for German ones, using different phrasing, splitting sentences, changing tense or grammatical number, etc. I think our goal here is to produce natural sounding English sentences that convey as much of the flavor, nuance and intent of the original as possible. In most situations, that simply can not be done with literal word-for-word translations.
My own view is that since one of the stated aims of Duolingo is to "translate the web", we should be writing usable idiomatic English. I look on a lot of the translations in there that have just been done word-for-word and wonder if the people making them have actually read what they've written. If I do a translation and it doesn't feel like everyday English, I'll go back and rewrite it until it does, even to the extent of paraphrasing if I feel it gets the meaning across better.