I agree with hM23 that the planning phase of the Project is crucial. If you have a great plan, someone else can always execute it. If you have a bad plan, but people to execute, is that helpful? If I had to pick two, I would agree with Scott that Planning and Executing are the crucial aspects for a successful project. For our projects at Sloan involving medical devices, there will always be a planning meeting and then a Planning document that is signed off-on before the Project starts. It will take a few weeks/months to actually get the Planning document executed. I also think dag56 has a good point of highlighting scope creep, because that can be detrimental to Project. Staying within a clearly defined scope is crucial, especially if you are using outside contractors to build the device.