To be 'Derlethian' is to cling to the dichotomy that August Derleth tried to impose on Lovecraft's (and others) writing. Derleth was responsible for the 'Mythos' as is popularized today. Attempting to classify everything into tidy little packages for bite size consumption.
It works to a certain degree in portions of text, but fails in the larger sense of the greater work Lovecraft and others shared. Derleth is certainly no schmuck, but at the same time I disagree with his attempts at confining that which defies taxonomy and was never meant to be classified in such a way. To do so removes the one key feature that Lovecraft encouraged, and that was humans are at large meaningless and that we are incapable of correlating such cosmic revelations within the breadth of human understanding.
In game terms, this is necessary and works to confine certain agendas within the context of popular archetypes even though they may be quite incorrect in their associations at times. Some poetic license must be taken in order to maintain game mechanics and the implied balance.