So, they blew up a bomb and created an alternate timeline that for an entire season was totally consistent with being an alternate timeline, including Desmond who we've seen since season 3 can travel through time serving as kind of an intermediary.

Then with four minutes to go they have Jack's dad say "no, this isn't an alternate timeline, it's the afterlife." and before we can say "that doesn't make sense!" there's a cast party and a cliche white light.

In my mind... those 2 minutes Jack's dad talked didn't happen. THEN the show has an ending thats happy, and doesnt' require a whole new level of reality to be added in the SECOND TO LAST SCENE OF THE FINALE.

The alternate timeline was created when they set off the nuke.
Desmond, who's electromagnetically special, was able to mediate between the two.
When a character was 'awakened' in the side-timeline, they were able to live out in peace the rest of their life in this better, island-less world, with full memories of the original timeline.
Jack died on the island, and ALSO got to live out his life with Kate in the side-step timeline.
Sawyer escaped the island with Kate and Mile, but ALSO got to be reunited with Juliet.
With their memories restored, all the Losties reunite at the church and have a celebration. THE END.

If it was heaven/limbo all along, why did it have a sunken version of the island? Why did Juliet say 'the bomb worked?' What does Jack's dad know about anything, since apparently every time we ever saw him it was really the Smoke Monster? Nope. F that.