Harvest timing is most of it
Aromatic plants are picked at the point their oil content peaks, and it is a narrow window. Chamomile is cut when the white petals begin to reflex downward. Lavender goes just before full flower. Osmanthus has a two-week season that supplies the entire year. Miss it and the material is bulk rather than aromatic.
Drying is where quality is won or lost
Almost every botanical is shade-dried at low temperature, and the reason is simple: the aroma compounds are volatile and heat drives them off. Over-dried chamomile browns and loses the apple note. Over-dried mint smells of hay. Sun-drying is faster and cheaper and visibly darkens most material, which is why the better grades are dried in the dark.
Roasting, for grains and roots
Barley, buckwheat, dandelion, chicory and burdock are roasted rather than merely dried, and the roast is the whole of the flavour. Light roasting leaves a root bitter and a grain thin; dark roasting brings out caramel. It is the one step in this category with a comparable role to what roasting does in oolong.
Cutting and sifting
Whole material is cut and graded, and this is where most of the quality difference in a supermarket packet comes from. Whole chamomile flowers brew slower and rounder; the same flower milled to dust extracts fast and turns bitter. It is the loose-leaf-versus-teabag argument again, in a different plant.
What to look for
Colour that matches the fresh plant rather than a browned version of it. Whole recognisable parts rather than dust. A stated harvest year on anything aromatic. And an ingredient list where the named plant is actually first by weight.