I keep seeing Walt brought up as an issue that people, seemingly above everything else, demand to see explained.
The problem, something the writers already noted, is that at one point they were considering just how they might incorporate Walt back into the story, considering the actor has aged significantly.
That's the problem with child actors for a TV series that spans 6 years of real time but on the show is less than a year.
They could have another character try and explain him, but it would be a little awkward having his character explained as if important, and his character is nowhere to be found.
To incorporate him with the same actor, they'd have to pull some major plot trickery and that in of itself would likely alienate viewers all the same.
They kind of wrote themselves into a corner with that one.
But not every special character is going to get explanations
Think it's likely we will ever find out why Miles has the "power" he has? I doubt it. We might get an explanation for Hurley considering the dead people thing seems almost the same as the MiB situation.
Some mysteries are better served to just be the way it is. Others deserve attention. Especially if that specialness is actively engaged for the forward progression of critical scenarios. Say, take Desmond for example. If they don't explain Desmond, you'll see me get upset. Considering his specialness, what it is, is seemingly going to be critical for the success of someone... that's kind of important. Walt... well.. while Walt kind of act as a plot device for awhile, he isn't there anymore, and doesn't appear to be necessary for the Losties.
Hell, they could have done something to incorporate Walt as a package abducted by Widmore and quickly dispersed any issues with using the same actor.