The "M" logo for McDonalds probably stands for "McDonalds"
McDonalds
Well, take Mcdonalds for example. You know the big m sign??? well when you are looking for mcdonalds in the phone book or wherever, its easier to find the big m instead of the word mcdonalds in plain text. and its kinda like when people see your logo, they think of your company or whatever it is that you do, and then they might remember it better when they need it or something... Logo is the first step to brand a company. It give a face to the company. For example, the Mc D logo. It's just a logo. We know them today because of their successful branding campaign. That' what make them popular (beside the burger of course). But, the campaign are held together by the image of the M logo. Imagine a Mc D advertisement without the Big M sign. Can you? ;)
McDonalds
It can be a matter of opinion but it stands out and instantly recognisable!
Probally
The M of McDonalds.
slogan is a revolutionary word. while motto is an universally accepted phrase
Mcdonalds & red bull Xpedx's logo is another example. Look at BP's logo.
probably mcdonalds
motorola
Most brands have their letters in their logo, just not all of them, like DC, or Aeropostle, or GAP.