In angiosperms what is the single most important characteristic involved in their evolutionary development?

Question

In angiosperms what is the single most important characteristic involved in their evolutionary development?

Answer 1

I would say true flowers, double fertilization, true fruit, and vessels.

True flowers and vessels are the two "big ones."

Vessels are kind of iffy though because some gymnosperms such as the Gnetphytes also have vessels, although they may have different evolutionary origins.