Expression
Language is an articulate use of symbols.
A symbol is something expression uses as a reference to something else.
Articulation is when a complex symbol is built up from simple symbols, such as when we allude to a "black cat", an expression built from a noun ("cat") and an adjective ("black").
Soliloquy (monolog) is the basic use of language.
A soliloquy is not really a "conversation with oneself". The child soliloquises before conversing with their mother; this is often overlooked owing to misinterpretation of the fact that expression requires psychic interaction via thinking and action. And soliloquy continues into maturity, in some important forms.
Some of the following explanations apply to both conversation and soliloquy, but conversation is a distinct realm with its own page, so here I focus on soliloquy.
A proposition expresses a thought, while an allusion is an expression that expresses no thought.
For instance, a work of fiction is not really conveying information about anything. The author seems to make statements about his characters but really is alluding to something else. As to what allusions express, I have no opinion. As propositions express knowledge, maybe allusions express action. That would cover expression's two modal roots. I hope at some point to think this through.
Many of what follows holds for both propositions and allusions, but propositions pertain to the realm of knowledge so I focus here on allusions.
Allusive soliloquy is roughly what highfaluting mentions of "art" usually denote, though decoration may also be involved.
Themes
An expression can be about anything.
Allusive soliloquy about humanity is sometimes called "high art".
In that sense, the highest expressions of all are about the highest human concern, which is the self's disposition.
The place of such expressions among expressions corresponds to the place of ontology among cognitions.
About mere disposition little can be said, but disposition takes form in attitude and is elaborated into principles, and it is these that provide the material.
For instance, fiction can allude to such matters through the qualities and deeds of the characters.
Media
Prose is what we all routinely speak.
True verse involves both cosmetic (decorative) and semiotic (expressive) actions.
Prose that consists of one-sentence paragraphs, or that has arbitrary line feeds, sometimes passes for verse.
Poetry may be defined as allusive verse with high topic.
In music, the rhythm expresses the organic and ethoic rhythms of human life, such as heartbeat and walking, while the form may express higher realms.
Visual expressions such as painting and sculpture that merely "imitate nature" are propositional. Beyond that, they are allusive, either over and above the propositional content (in representational expression) or instead of it (in abstract expression).
Of course, a mere black square, displayed without context and arbitrarily claimed as art, is really neither abstract nor allusive. It is just a trivial representation of a black square. (If you compose an "Ode to a Black Square", that might be expressive, but the square itself is no more expressive than Keats' nightingale.)
Standards
The only inherently bad expression, whether word or image, is an opaque expression, clarity being the only standard to which expression as such is subject. The word "indecent" has no semantic content; it conveys that someone deplores an expression, but says nothing about why they deplore it.
However, an interlocutor's choice of expression, as they contribute to a conversation, is a sunomilic choice not a semiotic one, and should be genial; that is, an expression, however clear, is improperly deployed if put to surly use.