The ODI

Two of the biggest questions following the finale were "What Happened to Walt? (There were set pictures showing him on set) and "What Was the Plane Wreckage at the End During the Credits?"

Well here are some answers for you all:

Explanation About Walt and Plane Wreckage During Credits

Question about the ‘Lost’ finale: I thought we were supposed to see Walt one last time? Did something change? – Craig
You perhaps got wind of what ‘Lost’ boss Carlton Cuse said at last week’s Times Talk Live event: “You will actually see Malcolm [David Kelley], when all is said and done.” But he wasn’t necessarily referring to the series finale that was broadcast on Sunday. So yes, they did find a way to reintroduce (a grown) Walt, but you’ll have to wait for the Season 6 DVD extras.

My friends and I are debating the significance of the wreckage shown during the ‘Lost’ finale’s closing credits. Was that the plane Lapidus was flying everyone off the island on? Or was that Oceanic 815, meaning everyone died in the original crash? – Leah
It was C) Not quite any of the above. While many have sought to read something into the serene image of the Oceanic 815 wreckage, sources assure me that coda was chosen strictly to serve as a “buffer” between the finale’s mind-bending final act and the local newscast to come.

Source: Fancast