It is right, dictionaries label key also as adjective.The Free Dictionary has

• key decisions, the key element of the thesis; adjective, meaning of crucial importance, significant.

But I think such a label is questionable as so often as to labels of word class in dictionaries. key is no genuine adjective. You can't form comparative forms (key/keyer/keyest) and normally you don't use it predicatively (something is key).

I doubt that you often hear it in this latter use, and if so I would say such use is exceptional and I believe not standard.

"key" if used in adjective position as in "the key figure of the novel" keeps its character as a noun. It is used metaphorically, "the figure as important as they key to a door".

True, we can replace "key" by "crucial", but that does not mean that "key" has changed its word class. Labels of word class in dictionaries are to be considered with a critical eye. It is not rare that they lead astray.

http://www.thefreedictionary.com/key

It is right, dictionaries label key also as adjective.The Free Dictionary has

• key decisions, the key element of the thesis; adjective, meaning of crucial importance, significant.

But I think such a label is questionable as so often as to labels of word class in dictionaries. key is no genuine adjective. You can't form comparative forms (key/keyer/keyest) and normally you don't use it predicatively (something is key).

I doubt that you often hear it in this latter use, and if so I would say such use is exceptional and I believe not standard.

"key" if used in adjective position as in "the key figure of the novel" keeps its character as a noun. It is used metaphorically, "the figure as important as the key to a door".

True, we can replace "key" by "crucial", but that does not mean that "key" has changed its word class. Labels of word class in dictionaries are to be considered with a critical eye. It is not rare that they lead astray.

http://www.thefreedictionary.com/key