Essentially, trying to do too much all over the ice. Which has them taking more chances with the puck, puts them out of position defensively at times, leads to more turnovers, and far too often, leaves more open ice for opposing teams.
If the team had a tighter D system, with better puck support and focused on shutting down the middle of the ice consistently, then our D would develop more confidence in D coverage and making simpler plays. Right now, they're relying too heavily on anticipation and it's very susceptible to D breakdowns.
To boil it down, its the difference between the Edmonton Oilers of the past 5 years and the Oilers of this year. And to go one step further: rebuilds happen from the goalie out. But if the systems don't support the proper development of the Goalies and the D in their own zone, then rebuild may never translate into a successful NHL team even with all the talent in the world.