[gquere]

Technically bcrypt isn't a KDF.

[kragen]

Hmm, isn't it?

[Genbox]

It isn't. You cannot derive an arbitrary length key with bcrypt.

[sushibowl]

A key (hah!) property of key derivation functions is that they allow you to customize the key that you get out, mainly because you may need a specific length (e.g. 512 bits) for whatever encryption algorithm you're using. bcrypt lacks this functionality: you only ever get 192 bits of hash.