Operating Systems

Theoretically, if the same group of programmers write both the operating system, and the applications, they will function seamlessly.

The programmers would know all the features and tricks of both sets of software.

In reality, software code has become so large that no one is able to work on both the OS and applications in a timely fashion.

Chas

Control, you decide what the system can and cannot do. You also lock users into your system.
It lets your creativity reign. It locks out externals (viruses, hackers,...)It also means you don't get to share with anyone else. Now come out and playnice;)

The application will have as much control (and therefore speed, ram, etc.) of resources as the OS. Essentially, the application/OS will have no chance of doing anything other than what it is programmed for -- which can be a good thing, or a bad thing -- it can limit you as well.