You haven't done much asp.net development I take it?
Still surprised I got voted down for that, since C and Java are still correct.
In fact, it's what I hated about Java web development the most... You had to compile, package, and deploy every time you made a tiny change. It was a nightmare.
The Play framework uses its own copy of the Eclipse compiler to compile everything on the fly while it is running in development mode...so as you as you save your changes, you can reload your browser and see them immediately.