Knowing when and how to be professional is certainly a learned skill. Granted, the problem is typically that people are unprofessional in the wrong circumstance, but it can go both ways.
Knowing when to be professional can certainly feel like common sense to someone with strong social skills.
I did not start out in life with great social skills, and have spent a great deal of time and effort to get better. One of the things I spent a great deal of time actively working on was reading tones, is this conversation fun, professional, object oriented, et cetera.
It's not surprising MCM sees a high concentration of people who lack these social skills, we are at some fundamental level a gaming forum for teenagers after all.
I agree that being wordy helps no one.
Writing concisely is an important skill in business, one a lot of the community is missing.
I disagree that "nobody in the real world talks professionally" there are certainly circumstances in which speaking professionally is appropriate. Even with my friends, we will occasionally fall into a professional dialogue because the concise nature of professional speech can improve a conversations flow (like when discussing the new Star Wars movie or arguing about the future of a popular company).
It's all about the circumstance, sometimes, being professional can benefit a conversation, sometimes it hurts it, reading when to use which is the skill much of MCM lacks. Hopefully this thread brings enough attention to the issue that those who don't know how will put in the effort to learn.
The way you’re describing the word professional seems to emphasize that it is an extremely subjective word.
Does being professional mean to use formal tone? To sound smart?
Or does it mean to actually be competent at doing a job. To effectively process the task you were given with.
I would be inclined to agree with the latter over the former which feels to fit more with your post.
Of course it can be argued it’s a bit of both, or not, depending on your opinion.

