I have collected some quotes from various sources to help you consider the weight of each of your words. At the end, I include a poem of my own, "Uncoupled", which may help illustrate how word choice and tone can create inherent tension.
Diction: "Diction refers to the individual words in a poem and may be classified in several ways. A poem's level of diction can range from slang at one extreme to formal usage at the other, though in an age in which most poems use a level of diction that stays in the middle of the scale, ranging between conversational and standard levels, these distinctions are useful only when a poet is being self-consciously formal (perhaps for ironic effect) or going to the opposite extreme to imitate the language of the streets." (Poetry, p. 11)
Vocabulary: The group of words that a given subject can use or has at the ready. Diction demarcates the vocabulary a poem can access, as does the surface subject matter. Metaphors enlarge the vocabularies that are available to the poem (by bringing in unlike elements in relation).
Denotation: the literal meaning of a word.
Connotation: "The implied meaning or feel that some words have acquired[.]" (Poetry, p. 12)
"connotation: Any meaning suggested by the sound or the look of a word or associated, however remotely, with its usual specific meaning. […] The connotations of language thicken its ambiguities and also give it greater emotional weight, so that they may enrich its value for the poet." (Poetry Handbook, p. 37)
Word choice: Word choice is tightly related to diction. As every poem has a speaker, every speaker has a level of speaking (diction). The diction determines the types of words that will occur naturally in the poem.
Word choice also refers to synonyms (and antonyms), homonyms, and puns. Etymological roots of words often lend opportunities for deeply embedded puns.
The thesaurus is invaluable to the poet. At the same time, the thesaurus reveals lateral meanings that on the surface seem interchangeable but, due to their connotations, vary widely from each other in the context of a poem (as opposed to prose). In a poem, meaning is condensed. Word choice is imperative.
"There's hope for someone who can be embarrassed by poor word choice." (Walking Light, p. 99)
"Only if we have been compositionally sloppy do we have manifold choices at the end of a poem." (Walking Light, p. 129)
"[O]ur choices narrow with every word we put down, and […] these constrictions are opportunities for invention and virtuosity." (Walking Light, p. 187)
Syntax: "[T]he order of words in a sentence[.]" (Poetry, p. 12)
"[S]tructure in microcosm, which is to say, syntax[.]" (The Flexible Lyric, p. 102)
"[T]he syntax is trying to speed up the line, and the line is trying to slow down the syntax." (The Sounds of Poetry, p. 29)
"The line and the syntactical unit are not necessarily the same." (The Sounds of Poetry, p. 30)
"[T]he syntactical energy is like a physical act of meaning. When we say the poem aloud, it is a physical act of meaning." (The Sounds of Poetry, p. 39)
Tone: It is through tone that the reader is able to put the speaker's words into context.
"tone: That feature of a poem which shows the poet's attitude toward its theme, toward a speaker or a person addressed in the poem, and toward the reader. The formality or colloquialism of the vocabulary, its vagueness or precision, the simplicity or complexity of the style, the energy or languor of the rhythms, the character of the stanzaic pattern, the use or abuse of certain technical devices, all contribute to the tone." (Poetry Handbook, p. 185)
"[T]hese variations in tone, the speaker's implied attitude toward the words he or she says, depend primarily on vocal inflection." (Poetry, p. 19)
"From the volume, pitch, relative stress, pacing, and rhythmic pattern of the speech--even if the actual words are indistinct--we reconstruct the emotional content." (The Flexible Lyric, p. 78)
"[T]he exchange, the actual language, needs a context in order for its 'meaning' to be clear. […] [I]nflection [is] a context of sound[.]" (The Flexible Lyric, p. 79)
"[W]e register the tonality--provided by the sounds in diction, syntax, formal manipulation of rhythm, arrangement of vowels and consonants[.]" (The Flexible Lyric, p. 84)
"A single note cannot truly be said to have any emotional suggestiveness at all. Even two notes in conjunction barely suggest a mode . . . . It's only when the third note is added […] that a key or tonality is established and with it some specific color or character that makes a song different in different keys." (The Flexible Lyric, p. 84)
"If the same image, in the same situational context, can command different emotional implications, then images are not a dependable source of tone in a poem." (The Flexible Lyric, p. 89)
"[D]espite the importance of each of the elements contibuting to the overall 'tonality' of a poem, I have come to believe that the best poems, even discursive poems of narrative or argument, build their tonal content primarily through their sounds." (The Flexible Lyric, p. 89)
"[T]one is located most often, most dependably, in sound, a nondiscursive context apart from though simultaneous with the discursive information provided. […] [D]isregard of the music of the poem, or application of only its technical apparatus (such as fixed meter and rhyme scheme), particularly in poems already heavily discursive to mode and structure, leads to a loss or disturbance of tone, and with that loss comes a loss of clarity." (The Flexible Lyric, p. 90)
Irony: "Irony is the element of tone by which a poet may imply an attitude that is in fact contrary to what his words appear to say. […] Verbal irony is the conscious manipulation of tone by which the poet's actual attitude is the opposite of what he says. […] Verbal irony is also a conspicuous feature of verse satire, poetry that exists primarily to mock or ridicule, though often with serious intent." (Poetry, p. 19)
"[S]ituational irony [occurs when] the setting of the poem […] contains a built-in incongruity. […] Dramatic irony, the third type of irony, occurs when the personal of a poem is less aware of the full import of his or her words than is the reader . . . . Dramatic irony, as the term implies, is most often found in dramatic monologues, where the gap between the speaker's perception of the situation and the reader's may be wide indeed." (Poetry, p. 20)
The Flexible Lyric, Ellen Bryant Voigt.
The Life of Poetry, Muriel Rukeyser.
Poetry, Second Edition, R.S. Gwynn.
Poetry Handbook, Fourth Edition, Babette Deutsch.
The Sounds of Poetry, Robert Pinsky.
Walking Light, Stephen Dunn.
Someone mentions how the days have turned, no more
charcoal nights stretched like apparitions
from afternoon to dawn. You agree,
yet you also harbor hope for struggling
dusk. Twilight's doomed to dwindle further
into evening. No horoscope scoured
in pitch-black morning will change this.
Fog promises to come back into vogue.
Rain will dampen snow into nonexistence.
Birdsong threatens to wake you.
Today's assignment: Consider the subject of your poem. Write down all of the words that would be in its vocabulary. (These invariably will be nouns and verbs.) Contrast this list with the words that are currently present in your poem.
Further resources: Diction or word choice in poetry