What makes a good logo?

Milton Glaser’s I Love New York logo
When it comes to seeing a logo that makes you wonder, “Why didn’t I think of that?” what is it about the design that gives that impression?
My good friend Lee Newham taught me about five important design elements when it comes to brand identities. Iconic logos are:

1. Describable
2. Memorable
3. Effective without colour
4. Scalable i.e. work when just an inch in size
5. Relevant to the industry in question

Points one and two go hand-in-hand, because if you can’t describe what a logo looks like then how will you be able to remember it?
Point number three is important because colour is secondary to the shape and form. I always leave colour to the end of the design process, because if the mark doesn’t work in black only, no amount of colour will rescue it.
Point number four is vital for collateral, such as office stationery (pens, pin badges etc.) — all those little things that can easily be overlooked.
Lastly, the design must be relevant for the business it identifies. This is accomplished through indepth research into the industry involved, and helps to differentiate from closely associated competitors.
I’ve chosen a logo to illustrate the five points:

The above logo is for Open University (OU), “the UK’s leading distance-learning organisation.” You can view the OU website here, where you’ll see the design used in context (and much smaller in size).

There are a number of text layout variations, which gives greater freedom for those reproducing the logo in different formats. For example, the top right mark (above) wouldn’t fit on the side of a pen as well as the centre right version (above).
What I enjoy most about this design is the simplicity (the ‘O’ inside the ‘U’). The OU logo has evolved over the years, and didn’t always have the ‘glass’ effect—a common trend amongst today’s logos. You can read more about logo trends here. It’s important to remember, however, that trends don’t last, and by designing using the latest fad, your logo will become dated, fast.
Take a look at how the OU logo appeared in the past (below).

The typography leaves a little to be desired, but the same dinstinctive, memorable, scalable, describable, reproducable mark was used to set the Open University apart from its competitors.
Do you have a favourite logo that uses these five universal elements? Perhaps there’s another factor you think should be shown.