I watch every episode of Heroes and am suitably engaged but also increasingly distanced.
I feel that as the crisis has intensified, the characters' personalities have shriveled up a bit. They're alternatingly hopeful or serious, occasionally relieved, but otherwise more and more drained of color.
It's not that I've stopped caring about the heroes or their mission, but I feel the plot has smothered the characters. They feel more like cogs in a machine - breathing cogs, but cogs nonetheless - than people whose company I truly savor. They're not particularly exciting individuals.
Conversely, on Lost, I feel that no matter how the plot rises and falls, few if any of the characters lose their snap.