I want coffee.
I want some coffee.
Does these two sentences completely identical?
In general, is it possible to delete the word "some" from every instance of "some coffee", and to keep the same meaning?
Let's consider the following:
What would you like, tea or coffee? - I want coffee.
I'm feeling rather tired. I need (want) some coffee.
So, the question here is whether or not you can skip "some" in such sentences. As we can see, it really depends on the context. In a coffee shop, usually it's "a cup of coffee". "Can I have some coffee?" is standard in a party environment. Without "some", the sentence may sound awful in some cases.
In answer to your second query: Exception .1.
We like coffee does not mean the same as
We like some coffee. (implies some other is redolent of auspice, to quote WS)
however, We would like coffee //would like some coffee (idiomatic and equivalent )
Some coffee is lightly roasted. is not the same as
Coffee is lightly roasted.
Some coffee is better than others. (cannot lose 'some')
whereas (Some) coffee is better than nothing. (both are idiomatic & equivalent)
You'd most likely use "some coffee" when there is a pot of coffee to take from, and you'd like some of it. If you're at work and can't function, so you're looking for something to help you wake up, you might just say, "I want coffee." But you could you either one in either of those situations and no one would criticize.