Emeditor can show that a line has been changed by showing the line number in the left margin as bold. I think it would a nice feature with a color indicator in the left margin to show changed/unchanged lines like in Visual Studio 2010. It's much easier to see compared to the bold line number.

An example of the color indicator can be shown in the third image using the link below:
http://blogs.msdn.com/jasonz/archive/2009/02/20/a-new-look-for-visual-studio-2010.aspx

Visual Studio 2010 is just one example and i have seen the same kind of color indicator in other editors as well.

-gan