Light would actually fail, I thought it was within the bounds of the show to actually give us an ending where he succeeded in defeating Near.[spoiler/] Maybe that makes me more gullible than some of you, or less perceptive, but I have a tendency to let a story unfold for me at its own pace instead of looking ahead to where it will conclude; it drives my girlfriend crazy, because I'll comment on how something was really cool, and she'll make a snide remark about already figuring that out thirty minutes ago.
Anyway, I thought that the ending was one of the best parts of the show. It was exactly what I wanted.
Throughout the series, Light is put into situations where he needs to use other people to keep up his charade, and they eventually fuck up. I felt early on that this reliance on others was Light's greatest weakness. He can't directly control people (except in those situations when using the Death Note allows him to) and every person he brought into his cabal would be someone who could punch a hole right through his strategy with a little independent action. So I felt that his loss to Near was real and it delivered on things the show set up previously. I also felt that his breakdown at the end was believable. Light is a narcissist, a real mental disorder. Despite his brilliance and his ability to plan carefully, he is probably insane. The character reminds me a great deal of Hannibal Lecter. And when Light's plans fall apart and his weaknesses are laid bare, he cracks. His ego and character are monumental, and we should expect his fall to similarly great.
Despite all that, I am still digesting the series; I want to put it on my all-time favorite list, but I am waiting until I've answered more questions for myself. The rivalry between Light and L, however, is one of my very favorite character dynamics, and it was a blast watching it. One more note: for those of you who watched the new Battlestar Galactica (and I do not feel like getting into a discussion of that show's greatness or lack thereof) Alessandro Juliani, who played Felix Gaeta, does the English voice-acting for L.
EDIT: My browser is not showing spoiler tags, just the code for inserting them along with the material inside of them. I've tried to insert them myself, but I can't actually see if they work. Sorry if they don't.