Essential things in writing rap lyrics

Hi everyone!

I'm a music producer and songwriter. I've written lots of lyrics for pop, rock and metal songs, some rap beats, too, for myself and for hire, and now I'd like to try myself in writing rap lyrics. So my question is for experienced rappers here:

What is essential in writing a good rap lyrics? I've got the general idea how the lyrics are built up, but maybe I'm missing something crucial, so I'd like your advice on that.