This advice, as with much other good advice, should be taken and understood in context. It is an essay about how to write a good essay, and there are times when you want to break some of these rules.
As one example, expressions like "I think that" and "in my opinion" can be useful if used purposefully. When reading an argument, we have a tendency to fixate on the points that we take issue with personally. By saying "I think X is true" instead of just "X is true", you make it easier for someone to disagree with your specific point rather than your entire argument. In some contexts (like HN comments), this can help to reduce animosity and get much more of your point across to people who don't fully agree with you.
That's the point. If something is an established fact, I simply state it. Often these are uncontroversial things that have been demonstrated in some way. If I qualify my statement, that's because it deserves qualification. "I think" implies "I believe this to be true, but I do not know it to be true." So, yes, I lack total confidence in such statements.
It's also a way to differentiate objective statements from subjective statements: I think Metallica is a great band.
No, because the 'I think' does not mean what you think it means. You can't take it out of its social context, which is to inform people that you assign a lower certainty to the next thing you are going to say. (Of course, you could say all that, but that would make most conversations very pointless and annoying, as I can say from personal experience.)
You could say "I think the economy will be the most important issue in the next election", but you could also say "The economy will probably be the most important issue in the next election," which sounds clearer, as well as less refutable.
Because how are people supposed to know you are making a subjective/qualitative judgement? It's too easy to mislead people, and too many people profit from taking advantage of that.
They know by context. "Metallica is a great band" is obviously a subjective opinion and unprovable. It's the same as "I think Metallica is a great band" but with fewer useless words. This isn't misleading.
If I say something potentially provable that I am unsure of but claim is a fact ("Metallica is the best-selling band of all time"), this would be misleading, and I would be taking advantage of my audience.
There are other ways to achieve the same distinction. Consider "I conclude / posit / propose that ..." In general though, that distinction can be made from context. I would usually assume that content is an original thought unless stated otherwise, e.g. with a citation.
I go for "I assert" because it sounds aggressive, but it shows that I know that I haven't made my case yet. In the case of "Metallica is a great band," I'd just say it, though. Musical taste is obviously subjective.
I wish that were true, but it's not. People will flame mildly stated opinions as much as they will blunt statements. I think your point is more valid in less technical contexts, where being blunt can cause personal offense. But around here, the offense is a given -- it's driven by the technical content, not the tone. You can be as deferential an Apple fan as you like and the Android posse will still attack.
On balance, I don't even think this is a bad thing. Keep it short.
Well put. However, one suggestion of his, as related to college writing, that I feel is always valid is throwing away the common ideas.
The best advice I ever received for writing essays was very similar to what the author suggested. Every essay I write, before I begin, I come up with five ideas, or approaches to a subject or argument, and then I cross all of those off the list of things that I will use.
Since you can't hide within the "common thought floating in the community soup," it really does force you to tackle a direct, and often in-depth line of argument.
It takes a little more work at the beginning, but ever since employing the practice, I've yet to get anything other than an A on an essay.
Grammar quizzes however.. Oh, lord.. If only there were a trick for those.. My high essay grades subsidize my lack of anything resembling grammatical competence.
As one example, expressions like "I think that" and "in my opinion" can be useful if used purposefully. When reading an argument, we have a tendency to fixate on the points that we take issue with personally. By saying "I think X is true" instead of just "X is true", you make it easier for someone to disagree with your specific point rather than your entire argument. In some contexts (like HN comments), this can help to reduce animosity and get much more of your point across to people who don't fully agree with you.