There has been much talk of questions left unanswered, in particular the nature of the light. The creators pretty clearly left it vague enough for people to draw their own conclusions. However, as with many of the other mysteries, we've been given plenty of hints to guide our thoughts.
There was an artistic choice made at the end of the series, when the majority of characters are gathered in the church. The doors open and we see light. I think that the visual similarity between the light on the island and the light in the church is intentional because they are the same.
The creators of the show wisely chose to not specify what the afterlife is or isn't. The afterlife means different things to different people and choosing one explanation as 'correct' for LOST would lessen it's meaning to viewers who don't share that belief. So while we aren't told definitively what the light is, we are guided to the idea that it is the afterlife, whatever our own personal belief of the afterlife happens to be.
The Island has many special places, the most important of which is the heart of the Island where the light dwells. Interestingly, Jacob never got to see the heart of the island but us (the viewers) did. Like many other places on the Island, it was full of Egyptian symbols. This doesn't necessarily mean that Egyptians 'built' the Island, just that they were on it at one point. So I wanted to take a little bit to look up what the Egyptians thought of the afterlife to see if I could piece together what the earliest residents of the island (we know about) thought.
Before I go further into this, let me just say this isn't meant to be a theory on what s the 'correct' way to view the light. It's an exercise in trying to figure out how the Egyptians, the earliest inhabitants of the Island that we know of, might have viewed the light.
Not being particularly learned in the subject of Egyptian mythology I did a quick internet search and found the concept of ka, ba and akh. Basically the ka and ba are two parts of the soul. When a person dies the ka leaves the body while the ba stays behind. The ba needs to let go of its attachment to the body so it can join with the ka and form an akh. From this perspective, the FS begins to take on an interesting meaning. The FS characters could be said to be the ka, which merged with their ba when they regained their memories, forming the akh, which was then able to move on to whatever lies beyond in the light.
Two notable characters are Desmond and the MIB. Desmond's ba was able to leave his body while it was still alive. MIB was very different. He was thrown into the light. This killed him, causing his ka to depart as normal but it also moved his ka beyond into the light. This occurred before his ba could let go of its attachment to the body, so his ba was trapped behind, never able to rejoin his ka, become an akh and go in the light. This could be a 'fate worse than death' for the Egyptians.
We know Jacob was the protector of the Island for ~2000 years. We don't know how long his 'mother' had the job for. Whether she was the original, or inherited it from one before her we don't know. Regardless it seems likely that she had some knowledge, either directly or as a hand-me-down, of what the Egyptians on the Island believed. There were also some heiroglyphics that seemed to depict smokie, although we know from Across the Sea that the heiroglyphics predate smokie. So maybe MIB wasn't the first smokie. Maybe the Egyptians had seen something similar before and the line about a fate worse than death was what got passed down to the Island protector by the time Across the Sea occurred.
We can also examine the 'immortality' of some of the characters on Lost through the notion of ka and ba. If one's ka became stuck in their body, that person wouldn't die. This might be an explanation of what happens in the water drinking ceremony and with Jacob's touch.
None of this is to say that the Egyptian mythology is the correct view of the light. However, I think we can safely say that this is how the Egyptian inhabitants of the island may have viewed it. One can certainly view the light and island through the lens of a modern religion as well. (We've certainly seen quite a few theories along these lines posted.) In many ways that's part of the beauty of the Lost finale. We are left with a sense of what the Island is and the light is in a manner that we can all make sense of within our own faith.