Is the rejection of the current pope, by sedavacantist, the only major difference between them and Catholics "in good standing"?
I am certainly no expert on sedevacantism, but I would say there is more to sedevacantism than rejection of the current pope, but the other issues are sorta interwoven. Sedevacantists do not accept the last four/five popes as true popes (considering them heretics) and reject the Vatican II Council, considering many of the Vatican II teachings to be inconsistent with historical Catholic theology.
As a summary point from The Aquinas Site states:
"'Vatican II' and its 'popes' have taught, adhered to, acted in accordance with, or failed to condemn a plethora of heresies, including religious liberty, universal salvation, the efficacy of non-Catholics sects for salvation, the blasphemy that Jews & Muslims worship the One True God, the evolution of dogma, etc. They have also destroyed the faith of tens of millions, and Karol Wojtyla ('John Paul II') describes this whole process as a 'new Pentecost. In other words, he thinks it is good, and wants the Holy Ghost to take the blame ('credit')."
Wikipedia lists sedevacantism as a subset of Traditionalist Catholics but sometimes it is difficult to know where to draw the line amongst the groups. The various Traditionalists have differing "sticking points", but an opposition to the "Novus Ordo" seems to be a common thread. Some Traditionalist Catholics are in communion with Rome, some are not, although the status of the relationship with Rome can be unclear making it difficult to ascertain who is or isn't in "good standing".
Case in point, the Society of St. Pius X (SSPX) claims to not be schismatic, yet a recent article outlines a reconciliation attempt by the Vatican with SSPX. Interestingly, the conditions of the reconciliation focus primarily around allegiance to the Pope:
"Vatican sources confirmed that the reconciliation proposal included the possibility of establishing a "personal prelature" or a similar canonical structure for the society, which would allow the society a certain autonomy...The conditions laid out by the Vatican were:
-- A commitment to a response that is proportionate to the generosity of the pope.
-- A commitment to avoid any public intervention that does not respect the person of the pope and that could "be negative for ecclesial charity."
-- A commitment to avoid "the pretext of a magisterium superior to the Holy Father" and to not present the society in opposition to the church.
-- A commitment to demonstrate the will to act honestly in full ecclesial communion and in respect of the pope's authority.
-- A commitment to respect the date, fixed for the end of June, to respond positively. This deadline is described as a "necessary condition" for the preparation for a reconciliation."
I have often thought that Rome is willing to allow a fair amount of disunity in thought as long as an outward unity to authority is maintained. This article seems consistent with that idea. It seems unlikely, though, that there will be any reconciliation with the sedevacantist subset of Traditionalists anytime soon.
And since we are on the subject, I came across an online version of The Ottaviani Intervention, a book that deals with the Novus Ordo issue and was recommended by Gerry Matatics when I heard him speak.
ADDED 6/29/2008: Gerry Matatics was on the Iron Sharpens Iron radio show in April discussing sedevacantism. The MP3 is available for download.