I'm not from the S&P tradition, but Judeo-Spanish. I figured I contribute nevertheless. From the top of my head:
-) Introductory song: The sections of the haggadah are sung in a nice song, i.e."Kadesh urhats, karpas, yahats..."
-) Ladino: The second night of the seder, the maggid is read in ladino. On both nights, the introductions of each "section" of the seder are made in ladino, i.e. Kadesh is introduced by "Salyendo de arvit kuando s'eskurese, inchiran los vazos de vino i diran kidush." The songs, ehad mi yodea and had gadya are sung in Ladino. For ehad mi yodea, some of the numbers differ than the hebrew in the ladino translation, i.e. two are Moshe + Aaron, not the luhot ha-berit, thirteen are the ikarim, and not the 13 middot, and etc.
-) Celery leaves are used as karpas. ("ojika del apyo")
-) It is customary to act out the exodus from Egypt, and make a big deal out of it: Prior to beginning maggid, the seder plate is taken away from the table. In my city, the leader puts the matsot from the seder plate in a cloth bag, swings them over his shoulder and reads the pasuk and a half:
וישא העם את־בצקו טרם יחמץ משארתם צררת בשמלתם על־שכמם :ובני־ישראל עשו כדבר משה
In certain other cities, the leader asks for the people who are participating in the seder for his shoes at this point in reference to the pasuk
וככה תאכלו אתו מתניכם חגרים נעליכם ברגליכם ומקלכם בידכם ואכלתם אתו בחפזון פסח הוא ליהוה
After this, the kids are made to enact the exodus from Egypt which mostly entails them going around the table carrying the matsot which were previously placed in a bag and answer certain questions, like "where are you coming from?", "where are you going?", "how is tonight different than all the other nights"
-) Ma nishtana is sung in unison.
-) Vinegar is spilled from the cup when the makot are being read, instead of wine, which symbolizes tears. This is done only by the leader, and people at the table refrain from looking at the leader doing this.
-) After the seder, the leader hands out a part of the matsa from the seder plate for them to carry out with them in the remainder of the year.
-) During Dayenu, the leader reads the first part of every sentence, after which the people respond in unison "Dayenu!".
-) Some people read Shir ha-Shirim after the seder.
-) Most parts of the hagadda are read in a tune which is not used for any other purpose in the rest of the year -- as far as I can tell.
-) No stealing the afikomin. It's hidden underneath the table cloth, and it stays there until the end of the meal.
I never participated in an Ashkenazi or any other kind of seder, so chances are high, there are bunch of differences that I'm omitting.