what makes a logo great?

Some advice from a graphic designer's blog:

When it comes to seeing a logo that makes you wonder, “Why didn’t I think of that?”, what exactly is it about the design that gives you that impression?

There are four elements that can be seen in every great logo:

1. It’s describable
2. It’s memorable
3. It’s effective without colour
4. It’s scalable i.e. effective when just an inch in size

Point 3 is important because colour is secondary to the shape. Adding colour to your logo should be left to the very end of the process, because if the mark doesn’t work in black only, no amount of colour will rescue the design.