Something that's come out of my previous post on Rebecca is something that sounds a bit paradoxical at first: If characters are well drawn, I don't care *what* happens to them. I mean, I care, right, but I'll follow them to wherever they end up going. I won't throw the book down or switch off the box in frustration because 'that character would never do that'. the story arc can be a massively disappointing tragedy, but that's fine if I believe in the characters.
Take any character in The Wire, but in this case, Bubbles. I believe in that world and that character so much that however his story turned out, it would have been excellent and true to itself. By contrast, if you've got shit characters, you bloody well better have a gripping plot (so you don't notice the hollow characters) or a feel-good ending (so at least you get _something_ out of the experience). And strangely enough, shit characters and feel good endings have a large correlation. of course, rich characters and happy endings are a great combination, but when you've got such great material, happiness doesn't have to be the only outcome. Tragic endings for a pile of broken dickhearts is a risky, rubbish, recipe for a story.
as a final example, take 'no country for old men'. Pretty much everything is ruined for everyone at the end, but again, it's ok for us as an audience because it's engaging. But then that's also part of the Coen brother's oft-used theme of the vagaries of fate and the inconsequence of our actions. Oh, and great characters.
Maybe the point is that 2d characters, we project more of ourselves onto - so we don't want to see them come a cropper? whilst 3d characters are believable, so it's okay for us to watch them crash and burn? just a thought.