The problem with LOST wasn't that it went on too long, but that the story we are introduced to at the beginning of the show is not the actual story. Every season or so they would have some big reveal that basically said "everything up until now was bullshit, HERE is the real story." When all was said and done, the final reveal was that, yet again, it was all bullshit.
No matter how good the character pieces were, no matter how interesting the flashbacks were, no matter how well any of it was done, the promised resolution was never going to be good enough. From about half way through I thought the end would be pants (I couldn't think of any way they could pull it off) and yes, it was pants.
The ride was great, but the destination was never going to live up to it. Shortening it would have reduced the amount of time I wasted on it, but not improved it...