And now it appears that the anti-Assad side has, in effect, rejected the conference by seeking preconditions it knows Syria's existing government will not accept.

Reuters reported:

Syria's opposition coalition said on Wednesday it would only take part in a planned peace conference in Geneva if a deadline was set for an internationally-guaranteed settlement based on President Bashar al-Assad leaving power...

The declaration said "the removal of the head of the regime and the security and military command" was paramount.

Syrian opposition says peace talks must guarantee Assad's exit | Reuters

The anti-Assad party should properly raise its position at the conference, not as a condition for attending the talks. The reality is that it does not have the battlefield position to impose its terms and the battlefield situation is very likely to be a material element that could shape the outcome of any conference. The magnitude of the differences between the parties might also preclude a firm deadline.

All said, on account of those differences, I don't expect any breakthroughs should the conference be held. Agreement on a ceasefire and possibly a modest agenda for a subsequent round of bilateral talks is probably about as far as things can currently progress. In terms of the anti-Assad group's end goal, a political path that leads to elections, perhaps to adopt a new constitution or to choose a new government, might offer a means for inducing Assad to allow himself to be eased from power (he has offered hints in that direction in the past, but his battlefield position has improved since then). Tangible guarantees of amnesty for him and for his senior officials might be required. Of course, a political path along those lines does not assure that a post-Assad Syria would be stable, liberal or democratic in nature.