I don't think many people consider the body of work of Lost to be great. I think that people think the first few seasons were great, before it became apparent that they didn't really have a plan and were just sort of making it up as they went. Ultimately they were unable to bring about any real closure to the various storylines. For a mystery show, that's a very real failure. It's like a whodunnit where the writers don't actually know who the culprit is and just implicate everyone before finally just picking one at random.