What do you think are the biggest mistakes that the writers made?

LOST is one of the best written shows of all time with an incredible plot which really does keep you entertained for all six seasons of the show. However, there are a few plot missteps made by the writers which the show would have been better off without.

I put together this short list of some of the things I consider to be plot missteps in the show such as when The Others abandon Otherville/Dharmaville and also the death of John Locke because I feel like his death was extremely unsatisfying since he never gets his beliefs affirmed (The Smoke Monster/Flocke does not make up for this).

What are some things in the show that you consider to be mistakes by the writers?