It is not an oxymoron. Do you have a job? How effective are comments like: "Bob is a douchebag," or "Jane ruined this office." Do you understand criticism? It has three parts: Analysis, Interpretation, and Evaluation. If criticism can include just two of these elements then it can be helpful and constructive. If posters say that a feature 'sucks' but don't offer any reasons why or suggestions for improvement then they are wasting their time, the developers time, and all of their reader's time. This is my problem. 25% of the comments in this thread are a waste of time. They are useless and hateful. They distract us from meaningful communication about Shafer's departure and what this could mean to civ5.
I am well aware of the definition of the cosntructive criticism and it's generally popular accepted usage. However, I tend to view all sort of "talk" ineffectual and ultimately irrelevant (especially on a platform like a fan forum which primary function is to, well, facilitate talks). The only relevant thing is what we "do".
An active feedback, the kind which you would prefer, is only effective if it is being fed to someone who has the power to DO things. So ultimately, the real effectiveness only lies with the ability to DO things. That will explain why you rationale with the office setting is not relevant here, because in an office environment all parties involve has the ability and responsibility to act (i.e. DO things) instead of just talk.
Also, a constructive feedback is only so when it is actively sought after and asked for by the same people who can DO something. Otherwise, it's all just wild talk and people with the ability to DO something might not even be receptive to all your "feedback".
So here's the million dollar question, are you one of the people who can DO anything with ciV? If not, on what grounds are you asking for constructive feedback?
On forums, or most of them anyway, asking for and providing constructive feedback more often than not is just a way for poster to sound mature and intellectual, while in actual fact they are seldom either of those things.
So the simple answer is (if you skipped the complex part above), constructive feedback is as effective as "vitriol" in this particular environment, and I will stick to my favorite brand of sulfuric acid, unless I see one of the developer asking for constructive feedback.