Our local Ellis Hollow blog has some nice poinsettia pics. As I mentioned earlier, a plant disease is responsible for bushy, heavily-flowered (well, they're not really flowers) varieties of poinsettia.
Phytoplasma infections commonly cause symptoms of phyllody (the transformation of flowers into leaf-like structures) and virescence (the production of green flowers). Much like virus-induced tulip color breaks*, phytoplasma infection of poinsettias is commercially valuable.
Modern tulip color breaks (irregular stripes) are actually caused by a genetic mutation (not viral infection), which is much better for long term health of the plant. I don't know if similar genetic improvements have been made to poinsettias.
*The most famous color break tulip, Semper Augustus (pic), marked the peak of the speculative bubble known as tulipomania. In one famous account, a single tulip bulb was supposedly sold for "two lasts of wheat, four lasts of rye, four fat oxen, eight fat swine, twelve fat sheep, two hogsheads of wine, four tuns of beer, two tons of butter, one thousand pounds of cheese, a bed, a suit and a silver cup!"