The thing most resonates with me in stoicism is the fatalistic outlook. Everything that happens, has already happened and will continue to happen. Eases my anxiety.
The negative visualization described in the book is also a powerful technique. It puts one's petty problems into perspective and actually makes you grateful for things you have.
Overall, I think the central idea of it is the ability to feel joy - something that can be derived from the state of tranquility rather than pleasure of the senses. And of course, to treat other people to the best of your abilities and at the same time acknowledging how flawed beings we are.