I fell asleep during the season 3 finale of LOST and didn't get to see my girlfriend's reaction. I'm so pissed!
I remember watching that with my dad and just absolutely losing my mind. The wait until season 4 was so painful that I actually bought the first episode [of season 4] on iTunes as soon as it was available so I could watch it immediately instead of having to wait for the rerun. I can't think of a single show since LOST that has grabbed people the way it did.
Man I love this show.