Hi I'm a copywriter so very used to written briefs. Try to keep it to one page if you can, but 1.5 at the absolute most.
Key area to cover are:
:: Background info on who the logo is for
:: What the logo will be used for (ie to make sure it will work across all media - internet, print, stationary etc) also, would it be helpful if the logo could be animated?
:: why the logo is needed (whether it's a new company, or a re-brand)
::Preferences on typefaces, colours (one colour, lots of colours etc), imagery - be specific about what is NOT allowed
:: think about the tone of the logo - serious, business-like, funky, friendly, approachable
:: state target audience - mums, families, teenagers etc
:: What you want people to get from the logo - think of this as a single-minded proposition ie. this company is the no.1 resource for savvy mums
:: include a short list of words that help describe the company (ie dynamic, friendly, funky etc) in terms of it's personality
Can't think of much else of the top of my head - sorry I don't have an example I can link to. But hope that helps a little!