A theory I read somewhere holds that on any topic, there are only two things that matter. Everything else is a side effect or a result of those two things. Hence, for management consulting, my two things are:
1. Without good implementation, the best ideas are useless.
2. At some point the client takes over.
This is another way of saying that whatever you do must work in the real world - the greatest idea is nothing if the business can't make it happen. This is one reason why the best ideas are sometimes the simplest.
