blah blah blah elitist blather blah blah blah
As long as we're going to be prescriptivists, it's wise to at least be "correct". Let's take a glance at the original sentence, shall we?
It's one of those words that just suck.
The above, even to the most thorough prescriptivist's eye, is correct.
It = Subject
"is" = Verb, truncated into the contraction "it's"
one = Object
of those words = prepositional phrase describing object
that just suck = restrictive clause, describing previous prepositional phrase.
As a general rule of thumb, when in doubt, remove phrases that don't change the core meaning.
Let's do that, shall we?
It's one of those words that just suck.
We'll untruncate the contraction.
It is one of those words that just suck.
We'll take out the "just", as it's an adjective adding precious little meaning to the sentence.
It is one of those words that suck.
That's the most this sentence can be simplified without drastically altering its meaning. Were the adverbial phrase
"of those words" removed, the subordinate restrictive clause
"that suck" would be modifying the original phrase
"it is one", in which case the verb "suck" would need to agree with its (new) subject "it".
That's not the case.
You're also mistaken in labelling "that" as a relative pronoun. In this case, the word is working as a conjunction, joining the verb "suck" to the adverbial phrase "of those words".
I think I can illustrate this in a way that's more transparent. Rather than stripping the sentence down, let's reorganize it. Adverbial phrases are, after all, very mobile, and can often be moved without changing the inherent meaning of a sentence. By changing the order of clauses, we can more clearly see the meaning of each.
Of those words that just suck, it's one .
This retains a nearly identical meaning, but is certainly phrased in a less than ideal fashion. It is clearly distinct from the way in which you were mis-correcting the sentence, however. Let's see what happens when we dislocate its adverbial phrase in your version.
Of those words, it's one that just sucks."
This clearly expresses an idea of "it" being distinct from the super-set "those words", as one that sucks. That's clearly not the original intent of the sentence. The original author was trying to point out that the "it" is a member of the group "those words that just suck". Thus, the original sentence was, as previously stated, entirely correct.
All of that being said, don't correct people's grammar. Your grammarian certificate has been revoked. You're not trying to teach them anything. You're trying to prove that you're better than them, and no one likes that. Rules of grammar are highly plastic, and anyone who believes in their strict application is surely deluded, and almost entirely ignorant of the nature of the english language.
Thank you and good night.
