I'm interested in knowing what are the most important features you look for in a logger? Should it have colored output, should it allow multiple output channels...

PD: I know it depends on the task you are approaching, but let's generalize for a bit :P

I currently maintain a Loggin' library for NodeJS and for the browser (partially), and wanted to gather some information on what features people want the most in a logger, as a way of improving the library.