I personally think Elend was too much of a fool when it came to his mistakes in book 2, but he was quite interesting in book 1 when he'd sit with Vin and read. I loved his development of leadership skills, but Tindwyl dying sucked. I liked Sazed but I couldn't completely relate to him. Vin was so special because she was good at everything and she was like a cat: independent, solitary, quick on her feet, wary, smart... Out of all the Mr S books, I'd have to be unoriginal and say that Lightsong was my favorite, but I really, really liked Sarene too, and if I'm not mistaken, she's supposedly the least favorite of the three main characters in Elantris.
I'm not normally a romance person--it's always good as a side-plot but never as the main plot--but the epitome of good character development is when you can trick the reader into falling in love with both characters who fall in love with each other, and Mr S has frustratingly done this to me with every book and it just melts me. Siri coming to realize that Susebron wasn't as evil as the priests had painted him to be and her befriending of him, and eventually her falling for him, was one of the most magical stories I've ever bought into. Mr S said he wrote Warbreaker during a very happy time in his life (when he got married to Ms S) and I really think that feeling sank heavily into that one work.