Even if you hate Lost's ending, which I very emphatically don't (Across the Sea was stupid, yeah, but meh), I don't understand how that can, for example, possibly invalidate the first three seasons' greatness. They have nothing to do with each other.
The problem with a show leaving its central conflict mysterious is that, were it turn out to be weak or disappointing, it would retroactively undermine the seriousness and significance of the earlier part of the plot. That's exactly what happened here.
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et tu, Joey Crawford?
http://i.imgur.com/HuR88.gif