What is the most significant thing you learned as a Commander?

Trust in your NCO's. Praise in public scold in private and try to be understanding to the younger Service members. I got lucky and had a very strong NCOIC I passed along what needed to be done and let the job get done . Supported the decisions they made and it made my 2 years fly by.

If you take care of your people and keep your seniors up the line out of their business, your people will take care of you and make you look good

An often forgotten piece of commanding - your Soldiers have family members. You should have an FRG already going when you take command, but in either case, get some training as soon as possible (heck, before the change of command) from Army Community Service. Cranky family members lead to cranky Soldiers, which leads to even more problems.