Under promise, over deliver.

One of the most disappointing things in project delivery is when you fail to meet expectations. Everyone was hoping for success. The definition of success was set. But the team failed to deliver on all of the goals. They may have worked hard, they may have delivered a ton of product, but since they didn’t meet the expected goal, they are considered to have failed. Everyone is let down.
Many projects are set up to fail in this manner. Stupid, ignorant managers believe in “stretch goals,” believing if they just keep asking for more, they will get more. In the end, all they ever get is a failure to meet impossible expectations. Morale plummets. People leave to find a purpose in life where they can feel like they are achieving something worthwhile. They go to seek jobs where they have a chance to win.
Now let’s compare this to a team that has reasonable, or lower expectations. The team works hard, delivers a lot of product, same as before. The difference here is that the team delivered more than expected. Everyone is pleasantly surprised. People feel amazing. There are celebrations. Confidence builds, and the now encouraged team endeavors to take on the next goal.
It’s a night-and-day difference, and sometimes the biggest difference between the two situations is simply the expectations.
Wouldn’t you rather be awesome?
Thus, one of the most important jobs for a manager is to manage expectations. It can be a very difficult task at times, especially when you have counterparts in the business who aren’t in touch with reality. It also requires you to be able to predict and see the future, which is always fraught with peril, as there are always unforeseen challenges. No one said it would be easy.