As far as I know mitzvot such as eating marror or saying hallel at the Pesach seder are only rabbinic (the marror was biblical previously but I am told that is no longer so). If this is the case and ...
During the Pesach seder, but not during the rest of Pesach, we say b'rachot for the mitzvot of matzah and maror. (I mean the "asher kidshanu b'mitzvotav" b'rachot.) So it seems that we are only ...