First, there are factors which seem to be purely pass/fail; missing even one of them flunks the Reputation check:
- Shepard must have at least four bars of Reputation.
- Tali and Legion must both be present.
- The player must have imported a save from Mass Effect 2.
- You must save Admiral Koris
Additionally, there are more factors which build up Shepard's trustworthiness to both parties; if Shepard has not done enough of them, s/he will not have the groundswell of goodwill and be unable to moderate a cease-fire. They are:
- Destroying or rewriting the heretic geth in Legion: A House Divided. In shipboard conversation, you can ask Legion about the repercussions of your decision, and it will mention either that the heretics were strongly in favor of allying with the "Old Machines," or that their absence made the consensus more difficult to achieve.
- Preventing Tali's exile in Tali: Treason. If you did not import a save, Tali will have been exiled and will not have be able to support Shepard with an Admiral's authority.
- Brokering a peace in the Tali/Legion loyalty argument. It is not currently known what Shepard's "canon" decision on the loyalty argument is (if any) if you did not import a save, nor if it impacts dialogue here.
- Completing Rannoch: Geth Fighter Squadrons. This may be a pass/fail factor, as players have reported being unable to access the compromise without it, despite having done much or all of the other activities. Legion references it during the conversation, asking the question that started the Morning War: "Does this unit have a soul?"