I am in a vague agreement with Draco, although I think the planet's typical themes can have more in common with one sign than another. Where I think the outer planets differ from the inner ones is that they have the ability to transform the sign they are in.
For example a Venus in detriment will have its essence in opposition to the sign in which it is placed, which will cause difficulty.
But a Neptune in detriment or exaltation will only mean that the Neptunian themes are similar to those of the sign. As Neptunian themes can be positive or negative detriment and exaltation are less useful as terminology.
The key question then is what is it about a Venusian essence that distinguishes it from a Neptunian theme? The outer planet themes are much broader and more social than the inner planet. The inner planets often manifest in material form- Venus is associated with beautiful things for example, or women etc. The outer planets have a much more transcendental and intangible manifestation- subconscious feelings of guilt, beauty and anger, that permeate throughout society and the individual. Violence is intangible- it is an action and its results can be seen but the planet and cause itself cannot.
That aside, I think that Neptune shares many themes with Leo rather than Cancer. Leo is all about the image, the personality, character, all things which are illusory. Cancer in contrast is the family, genes, blood ties and the home, which are all physical manifestations. Creativity and aesthetics in their highest forms are said to be divine or the more perfect manifestation of man. Neptune and Leo evoke a filmstar glamour such as Marilyn Monroe, or a fantasy world, of Hollywood etc.