'Potential' as an adjective

It seems that 'Potential' is one of a small group of adjectives that can in fact only be attributive (part of a noun phrase). Turns out, the dictionary does indicate this:

http://oxforddictionaries.com/definition/potential

Other examples are : maximum, outright, total (in one sense), utter...

The word "potential" has two common meanings as an adjective. One is "capable of being brought into existence but not yet realized". The other is "having capability or power".

For the former meaning, your usage is correct. For example, if you wanted to express that a "potential client" was actually quite a long shot, you could describe the client as, "very potential". That is, the attribute of being merely a potential client applies to him to a greater degree than the usual potential client.

For the latter usage, you have to follow the word "potential" immediately with what the potential is. Only the adjective usage permits "potential" to be used alone.

He has a lot of potential.

He's a potential superstar.

There's potential to this question, although my response may lack potential to adequately answer your question! :)

This is a very good question. I think in the way you are describing the use of the word, it would fall under a "quantifier pro-form". See Wiki on Pro-forms

In your example, you could define the use of the word 'potential' as a pro-adjective, in that it is a word used to describe a function of an individual such as "John Smith has a lot of potential when it comes to accounting". A quantifying pro-adjective surely fits the example because it is used to quantify John Smith's ability as an accountant.

According to Wiktionary, the problem is that potential is not a comparable adjective, it's an absolute - i.e., something either has potential, or it doesn't. Similar adjectives include extra, favourite and fundamental (there's a good list here).

Strictly speaking, even using comparative words like great or more or less are meaningless, because the degree of possibility is irrelevant - again, it either has potential or it doesn't. However, this is a rule that is frequently bent, as seen in great potential or more potential or less potential.

Let's try taking out the intensifier to see what we have left:

He is potential.

And there, we see the root of the problem, as clearly your friend wasn't trying to say that a person is the definition of potential, only that he has the quality of it.