In addition to leavening, there are five grains that are forbidden (Chametz) during Passover:
wheat, barley, spelt, rye and oats.
Also, because of the way things were done in the Middle Ages, at markets, how foods were stored, Ashkenazim aren't allowed to eat:
Corn, rice, peas, lentils, and beans. Many also include peanuts. This list varies by community, and doesn't usually apply to other traditions, like Sephardim or Yemenite Jews. Part of the reason is that in the Middle Ages, when the sales person was giving someone flour, the flour might drop into the barrel with those items. Another is that those items swell in the cooking...depends who you ask.
Many Ashkenazim are leaving that tradition, because it simply no longer applies.
Example: How does corn syrup swell? or corn oil? Or peanuts?
The term flourless cake doesn't refer to a cake made with Matzah meal substitute...it refers to a cake with no grains in it at all...I made one once. It was like eating a huge bar of chocolate! A bit heavy for my taste! Not doing that recipe again!