Lost was a sow that as it went on it created new layers to wrap around the original story of a plane wreck on a deserted island. Plot-wise it was a maze that was constantly evolving and becoming more complex (weird), but it was done for the purpose of showcasing the characters. How would our rats run through this new, more complex maze? Surely, at some point the decision was made that as layers were added (especially the weirder ones) there was no possibility of collapsing back to a point that could be explained. There are an unreasonable number of questions that have been raised to ever settle then within the confines of the show. Each "answer" was regressive because it posed more questions. The problem with the last season and maybe even the last two seasons, is that the weird plot overshadowed the characters. What weird thing can happen next replaced characterization.
To me, the sideways universe/flashsideways/alt universe/limbo construct was a way to focus on the characters again. There was no weirdness in the alt universe (except the underlying premise that it didn't exist). That plot line allowed us to see the characters that we knew act with the core of their true selves. The reason we cared about these characters in the sideways universe was because we had grown to care about them in the Lost world.
So, I kind of liked the sideways/reward/way-station universe sub plot. I thought it was OK. It is NOT a resolution of the questions of the island and in many ways was a diversion - a way to put the telling stories about characters genie back into the bottle and that was OK.
As for the Island main plot, I think that was more of a failure - too many layers of weird that were not developed or resolved (they shot a polar bear in the first episode for heaven's sake...). My one reservation with that assessment is Hurley and Ben - the odd couple protectors.
Side rant: Jacob brought people to the island 1: to replace him and 2: for "the game." It seems to be if he hadn't done #2, he wouldn't need #1.
