They have clear glass panes and sometimes, it drives me nuts when I imagine how people standing just outside our front door can peer inside and see all the way through our house.
I've been looking at different window-frosting solutions and found a roll of window film from DC Fix which you can use to cover glass panes and make them appear frosted - there's different designs available. Each roll cost me around $7, a roll covers up to 12 panes and each door as 15 panes - to save you a bit of calculation, I bought 3 rolls to cover both doors.
Now, when I hang out in the den and want the place to feel open, I just slide the doors open and when I want a bit more privacy, I close the doors.