The 80/20 rule or Pareto principle can be defined as 80% of the value comes from 20% of the features. Scrum focuses on building high priority items first, and questions the business value of incremental improvement. If it doesn't add value, why do you to build it?
Theory X & Theory Y
A key difference of scrum vs traditional project management, with scrum we actually trust the team to do their jobs. Traditional project management approaches projects with a command and control approach - the project manager and sponsor makes most of the decisions and doles out tasks to the team.
McGregor’s theory X and Y delve into human motivation and management techniques, in which theory X essentially states that workers dislike their work and have little inherent motivation and must be supervised to get their work done. Theory Y, on the other hand, eludes that people given the right environment and working conditions will perform well because they take pride in their work.
Scrum takes the theory Y approach and gives the team flexibility to make decisions which ultimately create ownership and a higher level of engagement in their work, all with superior results.
Defined vs. Empirical Planning
When doing something new – something for the first time, you must make certain assumptions to proceed. You don’t know what you don’t know. In a traditional waterfall project – you list out assumptions at the beginning and hope you covered everything. You then make a watertight plan and move forward blindly. That is defined planning.
With scrum you use an empirical approach – you plan based on what you know, what you have experienced. Experimentation is key.