There are two kinds of people using del.icio.us...

... taggers, and describers. I'm a describer. When I add a bookmark to del.icio.us, I add a description to it, mainly because adding a description is ridiculously easy -- it is simply a matter of selecting some text then pressing the del.icio.us bookmarklet.

I don't know what motivates taggers to tag -- you have to expend brainpower to come up with tags that make sense. I guess there are two kinds of del.icio.us users: those who tag, and those who describe.