A question which is often brought up at the seder is, why don't we say a bracha on the mitzvah of sippur yitzias mitzraim?
The Ritva on Messeches Pesachim (7b)adds that the reason the Rabbanan established that one should make a bracha before taking part in a mitzvah is to make it known that the reason we are preforming the mitzvos is because of our belief in the Ribbon Shel Olam. The Rosh while commenting on this Ritva says that there are some mitzvos that do not require a bracha because they, in their essence, serve the same purpose as the Bracha itself.
(Brief Tangent: A similar question is asked when discussing the topic of why there is no Bircas HaMitzvah before Krias Shema. The Avudraham elegantly answers that the entire purpose of a bracha is to be mikabel ol malchus shamayim. There is no reason to do this before taking part in Shema because being mikabel ol malchus shamayim is the who purpose of its mitzvah!)
Another answer to this famous question is offered by HaRav Dovid Oppenheim, Av Beis Din of Prauge. The Gemara in Megilla (18a) says that a person should not participate in praising Hkb"h more than normal. The Marsh"a while commenting on this offers his explanation of the Talmud Megilla by saying that this only applies when it is in the context of a Bracha. (proof: Gemara Brachos 25a - we ask a person who says too many praises of Hkb"h, "Have you said enough?!?!") From this it is implied that if one excessively praises the Ribbono Shel Olam without the context of a bracha then it would be alright.
For this reason we do not say a bracha at the beginning of the Seder. The following discourse may occur at your Seder:
Head of the Seder - "We have a mitzvah to discuss Yitzias Mitzraim
Member of the Seder- "Why don't we precede this mitzvah with a bracha, like my all other mitzvos?"
Head of the Seder - "V'Chol Hamarbe Lisapper B'Yitzias Mitzraim Harei Zeh Mishubach - It is praiseworthy for one to be plentiful in the telling of the Exodus from Egypt." Therefore we cannot say a bracha, fore when we say a bracha we are not allowed to say an excessive amount of praise to Hkb"h. For this reason the Rabbanan never established that there would be a bracha on the mitzvah of sippur yetzias mitzraim. So that we can spend an entire seder singing out praises to the Ribbono Shel Olam for taking us out of Egypt!"