Thanks, Triangle, very interesting. My favourite Dylan Thomas poem is Fern Hill, http://www.poets.org/poetsorg/poem/fern-hill. He uses all the devices mentioned by Owen Sheers (with the possible exception of onomatopoeia?) in a quite subtle way, but also uses imagery drawn from all over the place (including the Bible) to paints marvellous and evocative pictures of the transitoriness of human life and the way we can respond to it.
'Time held me green and dying
Though I sang in my chains like the sea.'
Sums up the humanist attitude to mortality perfectly. Arguably even better than 'Rage, rage against the dying of the light.'