Having removed a couple death blocks myself from climbing routes while doing maintenance, I can tell you how scary these things are. They are very heavy, but someone using it as a hand hold can dislodge it very easily.

One block I rapped in on to remove was only being held in place by a stone about half the size of my hand. All I did was push that stone away and then lightly pull with a pry-bar and the entire block plummeted and destroyed the trail I had just performed maintenance on 2 months prior. Better the trail than a belayer or hiker. The block had chalk all over it, so people used it as part of the route. It had to go immediately.

If you are a climber and climb outside often, I highly encourage you to donate some money to the ASCA because they send people who do maintenance new bolts and equipment to make sure routes are safely climbable.