To answer this question, I'll use Thomas' poem "Do Not Go Gentle into That Good Night," as this is a very well-known poem where Thomas speaks to his dying father, telling him to die well and with with strength. If you read that poem aloud, there are several elements that aid its flow, rhythm and readability. I provided a link below of the poem itself.
First of all, Dylan makes great use of rhythm and rhyme. As you read the poem, it sounds like a song, with a lilting bounce and roll to it. He achieves this by using 10 syllables in each line; this provices a parallel structure that sounds planned and even, balanced and structured. Then, he rhymes words in an ABA structure; the ends of the A lines rhyme. This makes it sound like a rap, or a song, as those tend to rhyme. The rhythm and rhyme gives the poem an overall pleasing sound when read aloud.
Another technique that makes his poetry work when read aloud is the use of repetition. Think of it this way--when you are at a concert, and the chorus of the song arrives, the lead singer will often turn his mic to the audience and you them sing it. You guys know it, because it is repeated often, so are able to anticpate and sing along with that chrous. Thomas uses the same technique. He repeats the phrases "do not go gentle into that good night" and "rage, rage against the dying of the light" at the end of the stanzas; the audience can anticpate it, and speak along with him. That adds an element of anticipation and participation on the part of the audience, which is a nice effect.
Those three elements--rhythm, rhyme, and repetition--are all poetic elements that make Thomas' poetry great for reading aloud. I hope that those thoughts helped; good luck!
The poetry of Dylan Thomas is richly evocative of the sounds and sights of Nature - particularly those of his native country of Wales where the countryside meets the sea. In works such as 'Under Milk Wood' he also successfully evokes the mini stories and accents of the native Welsh people he used to hear around him. To do this, he bravely experimented with new effects and language forms, mixing simile, metaphor, adjective, noun, alliteration and onomatopaeia all together. 'The words 'slow-black, crow-black' in a describing situation is novel and eccentric but works beautifully, as does 'fishing-boat bobbing sea.'