Normally the sentence in question is analyzed as having a noun clause (content) as subject.  This makes the "correct" sentence:

	These personality traits is what prevents John from doing a better job.

Putting the clause first allows the plural noun phrase to stand as subject:  

	What prevent John from doing a better job are these personality traits.

The whole question of singular and plural agreement is often lost in present day colloquial English.  For example, the noun phrase with the pronoun "none" is usually taken as plural:  None of the passengers were killed. 
And yet, the phrase "no one" would be singular:  No one was killed.  

-----Original Message-----
From: Assembly for the Teaching of English Grammar [mailto:[log in to unmask]] On Behalf Of Roberto Perez
Sent: Thursday, July 09, 2009 10:05 AM
To: [log in to unmask]
Subject: "what prevent" or "what prevents"

Hello everyone,

I'm having a debate with a colleague regarding this sentence:

   These personality traits are what prevent John from doing a better job.

One of us says the verb ("prevent") is correct, because it agrees 
with "personality traits". The other one says it should be 
"prevents", because the verb should agree with "what".

During the debate a similar construction was used as a basis for discussion:

   These are the personality traits that prevent John from doing a better job.

In this case, we coincide that "personality traits" and "prevent" are 
in agreement, and that is correct. But the use of "what" in the 
original sentence changes things, because "that" only introduces a 
clause, while "what" is replacing a noun phrase, such as "the elements that".

Could anyone shed some light on the issue?

Thanks in advance,

Roberto Perez
[log in to unmask]

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