I loved the finale, no secret there. But a lot of people feel like the "afterlife" deal kind of had nothing to do with the Island and the first five seasons.
I feel like this special "afterlife" where they were given the chance to find each other before "moving on" is BECAUSE they were on the Island. After all, the Island is the most important thing, apparently, on the planet. The source/light/pool thing may be hard to understand, but nonetheless THAT is the key to everything and our Losties all played parts in protecting it from the one thing that could and had the will to destroy it, the Man in Black.
For that, they were rewarded with this flash-sideways afterlife so they could all move on together to the next stage of death. They were that important in protecting the Source and the Island (thus the world) that they were rewarded with the flash sideways timeline so they could stay together.
It looked like the "live together or die alone" thing was right. They all lived together to get to the end, so they all, kind of, got to die together too. They weren't alone in death and in moving on.
That's what I got from the ending, hopefully this helps soothe the pain a little bit more haha. Not saying I'm right, but that's how I see it. I will agree I would have liked to see concrete answers for the donkey wheel and the source and stuff, but what we needed to know is that it was essentially the most important thing to just about EVERYONE, which makes the Losties important people deserving of a chance to move on together.