What I think you did was have Mordin lead a team or go into the vents, he will die that way even if he is loyal. Mordin should never be used as a leader of the fire teams or for going through the vents. Otherwise, he died because he wasn't loyal. If everyone made it onto the Collector base alive and *they are loyal*, they will all survive no matter if you use the correct leaders for the other specialty teams.
Here's what to use:
On the infiltration, use either Legion, Tali or Kasumi (if you have the Kasumi: Stolen Memory DLC) for going through the vents. Use either Miranda, Jacob or Garrus to lead the Fire Team. Use anyone else you please with your character.
On the Long Walk,
Use either Miranda, Jacob or Garrus to lead the Fire Team, and then pick either Jack or Samara (or Morinth if she's is in your squad in place of Samara) for using their biotics for protection from the seeker swarms. Then choose anyone else you want to go with you. I found Mordin great for this part taking on the Husks, the Scion and Abominations.
[Edited by tommy6860, 2/12/2011 2:35:17 AM]