As far as the death of Vin and Elend...
Their powers are not greater than Ruin's. Anything they do that Ruin could anticipate, Ruin counters. There is one thing, one perfectly non-selfish act that could defeat Ruin. Ruin would never even consider martyrdom, because it is too selflessly outside his nature. He can't even fathom the concept, which is why they succeed.
Concerning the people who are disturbed by the death of their favorite characters...
I just spend 15 minutes trying to find the quote that goes something like... "If your goals are achievable in this life, then they're not big enough"... I don't think that necessarily holds true for everyone, but for Emperors and the like, especially Elend, it definitely applies. Elend and Vin are willing to sacrifice themselves for the ones they love, and for the greater good. Ghandi went out and did his work, knowing he'd received death threats, and died because of it. I think its small-minded and out of the nature of the book to challenge the death of the characters of the book.
Sacrifice is a reoccurring theme in the book, but especially in Elend's life. He sacrifices by leaving his comfort zone to engage in a real relationship with Vin. He puts aside his feelings of hurt and betrayal when the Lord Ruler is executing Skaa. He gives up his seat as King because of his ideals. He puts aside his ideals for the good of the Kingdom by becoming the Emperor. He eventually comes to a point where he can ensure the survival of the entire planet by keeping Ruin occupied with himself, knowing it will kill him.
If the teachers teaching high school have any brains at all, they'll have their kids read this book, and essays will be written about Elend's theme of sacrifice. (A Separate Peace really wasn't even a classic anyways
)
My question is... What were the 16% of the people who flat-out died to the mists?