Ok, I hope my experience by running through the last mission twice will be good enough to guide you guys.
Only loyalty matters
No matter who you pick your squad to face off the final boss, or escort, or tech, or biotic specialist, make sure they are loyal. Loyalty is the SINGLE MOST IMPORTANT FACTOR in getting the people to survive...it's isn't about whether or not Jack is a stronger biotic than Jacob. So yeah, as long as member is loyal, you can even pick a weak character like Miranda or Tali to be fire team leaders. They will survive if they are loyal.
Mordin
One other factor is Mordin. I think if you search around, you can find a lot of topics on how to make this guy survive. Quick version will be: Make him the escort and of course, make sure he's loyal. You can bring him into the final battle also, if he's your favorite battle team mate. That works too.
The Normandy
The other factor concerns your ship. Before getting through the omega 4 relay, make sure you do these 3 upgrades: Shields, Armor and Cannon for the Normandy. Otherwise when passing through the debris field, 3 lives will be lost, (or maybe 2...I'm not that sure. I just know that if you didn't upgrade, you will lose team members there).
The Normandy Crew - If you don't want to lose a single person
One factor will be regarding the Normandy crew. After the IFF mission, you must fly through the Omega 4 relay fast, preferably right after you come back on the Normandy. According to some discussions, you at most have time to do ONE more mission before the crew starts dying off. Crew will include: Kelly, Dr Chakwas, Engineers Ken and Daniels and others here and there. So if you have a lot of unfinished business, make sure you handle them before you get EDI to test the IFF.
Other factors which I may not be sure if it affects
Weapons and skill upgrades. If you have max out you rifle or shotgun etc. it seems that the character who uses the weapon will be harder to kill off when you send them off to other tasks.
Character class. Although now your team members doesn't have a distinct class, it seems it does matter whether there are enough people with assault rifles when defending the line at the final boss part. These 'soldier' members are supposed to be harder to kill. So if you find yourself having people dying off at tasks, pick these loyal 'soldiers' for the task and see how it turns out.
However, I can confirm that weapon and skills will not outweigh loyalty. Having weapon and skills upgraded but no loyalty will result in a higher probability that the character may not survive.
And to answer reaverblade50...I used Legion also for the vents, he survived.
Factors:
1. Loyalty
2. You must be fast in closing the vents. His health bar will be at the bottom. I feel that this part is tough (compared to the rest of the parts in the base) because the collectors are coming to slow you down. You must cut them down fast.
3. Sometimes after you close one vent, he will radio you saying that he's blocked again. The intervals between vents are uneven, so you have to LISTEN to him...and check your screen for the health bar popping up.
To illustrate my point that no one particular member is better at one post than the other, here's my rundown:
For Vents : Legion
Team 1: Jacob
Team 2: Miranda
Escort: Tali
Barrier: Samara
For Final boss: Thane, Jacob
All of them, including the Normandy crew, survived, and happily back. To be true, it isn't that hard to survive this mission, The developers make it sound difficult to come back from the mission. But if you done your homework, you will breeze through it and show the collectors who's boss.
[Edited by EverteMax, 2/4/2010 9:47:58 PM]