This one has been puzzling me for some time:
What is the difference in terms of usage between 關於 and 對於?
I know that there is one but I just can’t put my finger on exactly what it is.
This is an off-the-cuff answer, and is open to more discussion, but my gut feeling about it is that:
Dui4yu is used more for “in regard to” a certain matter, and usually is followed by details not internal to that matter, as in “In terms of dogs, I have no strong opinions one way or the other.” (Probably not the best example… 
)
GUAN1yu is used more for “about” as in “This is a book about dogs.”
I’m sure others can add to this or correct it further. What do y’all think?
I’d second that, and maybe add that dui4yu seems a little more formal than guan1yu, the latter being much more often used in conversation etc., I think … Xpet.
[quote=“Southpaw”]This one has been puzzling me for some time:
What is the difference in terms of usage between 關於 and 對於?
I know that there is one but I just can’t put my finger on exactly what it is.[/quote]
in general, “dui4yu” is more often used when someone wants to tell others his opinion; “guan1yu” is used for giving information. 
[quote=“Southpaw”]This one has been puzzling me for some time:
What is the difference in terms of usage between 關於 and 對於?
I know that there is one but I just can’t put my finger on exactly what it is.[/quote]
They both mean “about”, but it seems to me that “guan yu” tends to be more spoken and “dui yu” tends to be more written; similar to English “about” and “with respect to”.
Then there are other alternatives such as:
有關 X
就 X 而言
對 X 來說
(在) X 方面
all of which can mean just about any of the following:
with respect to X
regarding X
as for X
as to X
when it comes to X
vis-a-vis X
etc. etc. etc.
Be careful, though: in 95% of cases “about” doesn’t mean “with regard to”. This is only true in sentence-initial position when followed by a topic and then related information (“About last night, you see, what happened was actually…” 
) This is one of the 5% “abouts”, but most other ones would be more like “I saw an article about that on the Internet”. Definitely not “with regard to” in normal English, and I think there is a distinction to be made. The two phrases in Chinese are not interchangeable nor is the difference that one is strictly only oral and the other only written.