Both shows have an annoying kid, and benefit when he is featured less. I'm sorry, even though Henry (Jared S. Gilmore) can be a decent addition to certain episodes, he is also a drag on plenty of others. Lost kind of has a similar problem with Walt (Malcolm David Kelley), whom they promptly have kidnapped, and then send home. This wouldn't work with Henry, as Emma (Jennifer Morrison) would become just as obsessed with finding her son as Michael (Harold Perrineau) does, which would ruin her character on the show, and she isn't as expendable. Walt still appears from time to time throughout Lost, and it he ends up furthering the plot and becomes not as grating over time. But less is more with many child actors, and the writers would do well to remember that.
Both shows feature a flawed savior. Everyone believes in Jack (Matthew Fox) most of the time, but he isn't always capable of making the best decisions. He is reluctant to assume the leadership role, but it is something he naturally falls back on because he wants to save everyone. He also has made mistakes in the past. Emma is like Jack, trying to be a good person and do the right thing, despite what she may have previously done, and she has the ability to be a protector, unlike most other characters.
Both shows are willing to execute major characters, or send them away, leaving room to inject new blood, or expand a smaller role. Lost did this repeatedly, with some of the casualties including Boone (Ian Somerhalder), Charlie (Dominic Monaghan), and Libby (Cynthia Watros). Once Upon a Time, not quite halfway through season two, has already killed off Sheriff Graham (Jamie Dornan), had August (Eion Bailey) go missing, demoted Jiminey (Raphael Sbarge), and promoted Belle (de Ravin) and Ruby (Meghan Ory). But as any good Lost fan knows, characters can return at any time in the most unexpected places, so I wouldn't count out any of these people yet. Not even the deceased Sheriff Graham, because...
Both shows feature flashbacks heavily, and often. Everyone knows that a good way to learn about a character is to explore their past. But most television shows use this element sparingly. Lost and Once Upon a Time build entire episodes around such back stories, making them significant chunks of the episodes on a weekly basis, and tying the past to the present. When done right, it's clever and entertaining.