Traditional bones, illustrative flesh
Neo-traditional grew out of American Traditional in the late 80s and 90s, keeping the things that make Traditional work on skin (bold outlines, strong silhouettes, high readability) and expanding everything else. Line weights vary within a piece instead of staying uniform. The palette opens up from the classic five colours into jewel tones, purples, teals and earth tones. Shading gains depth and dimension, and the subject matter pulls in Art Nouveau, Art Deco and modern illustration influences.
It gets confused with new school, and they're not the same thing: new school is the 90s cartoon-graffiti look, exaggerated and bubbly, while neo-traditional is refined and decorative. It also isn't just 'traditional with more colours'; the illustrative composition and ornamental framing are their own craft, and the artists who do it well are specialists.
The classic motifs
Neo-traditional has a recognisable cast. Lady heads, often framed with florals, gems or headdresses, are the signature piece, broadly carrying the same beauty-and-mystique lineage as Traditional's pin-ups but rendered with far more decoration. Animal portraits are the other pillar: foxes, owls, wolves, tigers and snakes, usually front-on and ornamentalised, with the usual loose associations (owls with wisdom, wolves with loyalty) that you're free to take or leave. Florals, daggers, skulls and ornamental frames round out the set.