"Do Not Go Gentle into That Good Night" is harsh but lyrical, jarring but hypnotic. It's halfway between listening to monks chanting in Latin and listening to officers shouting orders at their troops. The repeated lines, called refrains, and the use of only two rhyming words give the poem a singsong quality. But Thomas also uses harsh consonant sounds, often alliterated, to give the poem an explosive feel. He also omits soft endings on words wherever he can – notice that his choice of "gentle" in the first line, instead of the more grammatically correct "gently," makes the word end on the strangling consonant "l" instead of the sweeter long "e" sound. The poem also has as few linking words and conjunctions as possible; connections happen through commas instead, as in "Rage, rage" and "Curse, bless." This means there are more stressed words in the poem, which adds to the feeling of a strong, intense rhythm.
For a taste of how the author wanted it to sound, check out Dylan Thomas reading "Do Not Go Gentle into That Good Night" aloud.