Unity is a multiplatform game development tool, designed from the start to ease creation. A fully integrated professional application, Unity just happens to contain the most powerful engine this side of a million dollars.
I have been programming in Unity3d for over 5 years now, and every year they add something new, something I or other developers needed. They aren't aiming to be overly complex, they are going to functionality and usability something that a lot of game engines fail to see, and fall just short of achieving but this is something Unity has not only excelled at, but has made it their goal.
Simple but Effective, that is what Unity is.