The rituals and forms of the Seder are very similar to the forms of the symposia, a type of Greek banquet which was wildly popular in the Hellenized world at exactly the time the Seder was created.
* Along with the food and wine, and essential feature of the symposia was learned debate and discussion. This, per Wikipedia, is why "the term symposium has come to refer to any event where multiple speeches are made."
* We lean at the Seder, as did participants at a Symposia. [Photo, and note the men are leaning to their left, just as we do] When the Sages said the point of the leaning was to imitate "free men", the "free men" they had in mind were the aristocratic guests at a symposia. If the Sages were to create the Seder today, in 2007, they might require tuxedos in imitation of the Oscars on the same grounds.
* We close each segment of the seder with a cup of wine. At the symposia the same custom was followed.
* Greeks and Romans both started their banquets with vegetables dipped in salt water, and charoset was a common dish (as implied in Pesachim 2:8 where it's suggested that charoset was served year round with flour)
Additional evidence of the relationship between a seder and a symposia:
* The Greek word epikomon means "after meal entertainment" and likely (again per Wikipedia) refers to the "games, songs, flute-girls, slaves performing various acts, and hired entertainments" that followed the discussion and the food. When the Sages said "one may not add an afikoman after the paschal lamb” they were referring to (and outlawing) this practice.
However, its crucial to remember that along with the blatant borrowing, the Sages also changed the content and purpose of the symposia. To me its quite obviously an atempt to take something secular and convert it into something holy. (NCSY, for example, often does the same thing.)
* The symposium were for wealthy, educated, men. In contrast, the seder was extremely egalitarian: men and women were required to participate together with their families, rich and poor alike.
* The Symposia was about studying, followed by reveling. The Seder is about remembering, rather than studying, and as noted above, the reveling was purposely outlawed.