Johnny Ringo said:

I'm just wondering if there's much to the fact that they focused on Some characters circa the flight but not others, ie: why Boon and not walt?

I'd think this would be pretty obvious. This is a TV show, and there are only so many elements within their control. It'd be hard (not to mention kind of lame) to find another actor that looks like the kid who played Walt did at that age, and they obviously can't use the same actor, as he is much older now (remember, this thing started back in 2004).

The reason they focused on Boone, was that they could get a hold of that actor (and it made a nice little moment where he tells Locke that if he were even in a survival situation, he'd want to stick around him). It would have been cool if they had brought back the entire season one and two cast. Would have loved to have seen Echo and Ana Lucia on the plane as well as many of the other characters we have long forgotten about, but it is understandable that they didn't.

There is also really no logical reason, that I can think of, why Shannon wouldn't be on the plane because of the destruction of the island, I am sure her absence had a lot to do with not being able/wanting to get the actress to come back, but they did a good job of using it as another way to show how things had changed this time.