Traditional Japanese Reiki is Reiki as it was originally practiced in Japan by the founder and his early students. This method of Reiki is unchanged form the original teachings and un influenced by Western or 'New Age' sources. Reiki was originally a very simple, practical and effective spiritual/healing system.
Some unique characteristics of Traditional Japanese Reiki compared to Modern Western styles of practice would be - the absence of the Indian chakra system, no invoking of spirit guides or dieties, use of only 4 traditional Reiki symbols, no crystals, no oils, no additions from other healing modalities. Traditional Japanese Reiki is just Reiki, plain and simple.