I lost Mordin simply because he can die really, REALLY easily if you leave him behind when going to fight the main boss. I simply took him along next time and everything was cool.
The choices weren't that hard to make, really. You know that Garrus commanded a good squad of his own, so he'd be a good team leader. You'd also know that Grunt would be a terrible team leader. For hacking the door, I can't understand why ANYONE would pick anyone other than Tali or Legion. One is a tech person, the other is basically a computer with legs. As for the biotics thingamajig, Jack or Samara is the logical choice, because the game explicitly states that they are really powerful biotics.
But the game doesn't say that Mordin has a high chance of dying if you don't take him with you. He doesn't always die, but he's got a chance of dying, which sucks. I immediately reset the game when he died, having saved just before the final choice. I pretty much refused to have everyone die.