To me, the FORM of the question is at least as important as the topic. That said, we should encourage topics that draw questions with good form and discourage questions in bad form (as defined below).
In school, we had two types of questions, identification (ID) and essay. (I am treating true-false and multiple choice questions as special cases of ID.) Good questions for the site are ID questions that draw factual answers. Essay questions are bad for the site because they lead to opinions, discussion, and debate, all the "bad stuff" we don't want here.
"What were the causes of the decline and fall of Rome?" is a bad question, because it is an essay question. Change it slightly to "What was Edward Gibbon's theory about the decline and fall of Rome?" and it becomes a good question because it is now an ID question.
As for answers, "Rome fell because most of her soldiers at the end were mercentaries rather than citizens," is a bad answer, because it is (yours truly's) uninformed opinion, and the first question i bad because it provokes such an answer. A better, factual assertion is "Edward Gibbons believed that Rome fell because most of her soldiers at the end were mercentaries rather than citizens," because the veracity of this can be checked by comparing it to what Gibbons actually wrote. Ths makes the second question "good." Or you could ask: True or false: "Edward Gibbons believed that Rome fell because most of her soldiers at the end were mercentaries rather than citizens." Another decent answer is "in year XXX, Y% of the Roman army was composed of citizens, and Z% of mercenaries."
Basically, this site was developed by computer programmers who want answers that have the property of "Turing completeness" http://en.wikipedia.org/wiki/Turing_completeness (have a definite stopping point), and questions that generate such answers, rather than questions that generate an" infinite loop" of answers. On this site, "not a real question" means likely answers will not be "Turing complete." And "not constructive" suggests the possibility for an "infinite loop" in the answers generated.
To sum up, good topics are those that generate "good" ("Turing complete," identification) answers, and bad topics are those that generate "bad," (an infinite loop of essay-type) answers.