I have four main objections that I will list moving from the least important to the most…

1. Many folk think that this is a good idea because it’s what Jesus did. See the post above—it’s not. The Seder liturgy as we know it does not date from the early first century period.
2. More often than not, doing a “Christian Seder” is disrespectful of Jewish beliefs. In essence, it can simply be an exercise in “playing Jewish” rather than attempt to honor and engage a related tradition; how would you like to hear about a Muslim “Easter Vigil”? I remember reading once—I can’t remember where—a statement by the rabbis on whether gentiles or outsiders can/should attend Jewish festivals. The answer was something to the effect of “If they have not mourned with us, neither should they rejoice with us.” In other words, if you’re going to do a Seder, why not also a Yom Kippur, or the fast commemorating the destruction of the Temple? Picking and choosing liturgical observances skews your sense of a tradition and what the celebration means (topic for later post: the modern insistence on the celebration of Sunday as a weekly remembrance of the Resurrection and the concomitant suppression of Friday as a weekly remembrance of the Passion. It seems to me you can’t/shouldn’t have one without the other…) The only way to do a Christian Seder with integrity is to do it in conjunction with a local synagogue. Their rabbis and leaders can make sure that the right things are taught/done/etc. LP—didn’t your former congregation do this? Even this, however, is problematic because I suggest that if the Jewish traditions and theology are properly honored, then—necessarily—the Christian ones are not.
3.  The point of a proper Jewish Seder (as I understand it and I’m open to correction if this is wrong) is the celebration of the Exodus. This is the Passover ritual. As a result, the meal remembers and celebrates the community’s salvation and liberation from Egypt and its formation as the freed people of God. What’s the point of Maundy Thursday, though? In the Christian tradition it serves as the first service of the Triduum—essentially a three day liturgy that begins on the evening of Maundy Thursday and that concludes with the first Mass of Easter. The celebration of the community’s liberation and salvation does not occur on Thursday for Christians, but Sunday. Some of the Easter Vigils that I’ve been to conclude with a big blow-out feast with a menu very reminiscent of a Seder; I think Anastasia’s church does/did this. That’s appropriate. But for us, a Thursday Seder celebration simply doesn’t work on the theological level. Consider for a second one of the major chronological differences between the gospels: the Synoptics (Matthew, Mark, Luke) have Jesus’ arrest after the Passover meal; John’s has it before so that the crucifixion coincides with the slaughtering of the Passover lambs. Historically, the Synoptics are probably right; theologically, John is.  
4. The practice of a Maundy Thursday Seder is symptomatic of a liturgical theology that I find very troubling because of its implications. It’s the notion of re-enactment. My students who advocated this practice were rather confused at my insistence that liturgy is not about re-enactment. Re-enactment suggests that we are trying to replicate something that happened in the past. The logic here is fundamentally historical—we are remembering a past event because the importance lies in the past. I was and am emphatic that liturgy is not about the past—it’s about the present and the future. We don’t re-enact, rather, we enact. We don’t celebrate the Eucharist because we are doing something from the past, but because in and through the Eucharist Christ is made truly and really present here and now in our very midst. While our celebration of Holy Week and Triduum is rooted in historical particularities, these particularities are not the principle focus. Rather, the present and future implications of those acts are what we experience and celebrate. Because Christ died, once for all time, we have been and are reconciled to the Father; because Christ rose, once for all time, we have the hope of resurrection and—indeed—experience foretastes of that resurrection even in our own flesh. These are not events that should be shoved into the past and re-enacted, but enacted and celebrated as breaking forth in our own time and place.