Now being with GE StackExchange for about two months, I observed that "opinion-based" as a reason for closing a question is applied quite quickly, too quickly from my viewpoint, and it is often mixed up with "off topic".

As an example, some time ago I asked: "Bringt der Import von 'impact' in den allgemeinen Sprachgebrauch einen semantischen Mehrwert? Oder handelt es sich nur um einen Akt konnotativer Aufwertung bzw., negativ ausgedrückt, sprachlicher Emigration?" This earned me a vote to close almost immediately as being too opinion-based. As one should look for the cause of such a verdict in oneself at first, the reason for that could well have been the abstract wording of the question as I just wanted to know whether "impact" in German can serve a solid semantic purpose – signifying something which hasn't been signified by a German equivalent so far – or only carries a different connotation (using an English term instead of a German one for reasons of style, or as a temporary fashion). I simply tried to bring up a denotation problem which can be discussed based on a well substantiated delineation of semantic fields, far from personal opinions.

> In fact, "personal opinions" often infect an answer with biased
> statements or even irrelevant chatter where it should be
> substatiated by facts and expertise. But then it is always difficult
> to determine if was the fault of the question being poorly put, or of
> the person who answered it.

As a rule of thumb, I think only such questions should be closed as opinion-based which require answers involving

1. Predictions ("How long will it take until I can speak German fluently"?)

2. Personal preferences ("Which German novel is best to read first for a beginner?")

3. Polls ("Findet ihr auch, dass Anglizismen zurückgedrängt werden sollten?")

4. Rankings ("What are the best methods to effectively learn German?" "I need a 'remote island' list of German literature")

Questions like "What is an effective way to learn German?" are, on the other hand, not opinon-based but off topic because the answers wouldn't apply to German only but to other languages as well. The point is that questions like this can be soundly answered based on scholarly research provided the answerer cares and is able to do so, but the answer would be not specifically related to the German language or require too broad an outline of the topic.