Have a think about the psychology of colours. What image of your company do you want to portray? Companies that provide services requiring trust may want to go for more conservative colours, like blue and green. Companies that want to portray themselves as young, fun and trendy might consider colours like red and yellow. Read up about the impact of colours on the mind, for instance here. Too many colours is normally not a good idea. Also think about accessibility, as a certain percentage of the population has colour blindness.
Just like for colours, what your logo actually depicts will have a psychological impact. Before commissioning a logo, sit down and think about some key words that you want to describe your company brand. It could be words like fun, vibrant, modern, quality, laid-back, young, classy etc... These are words that you need to give to your graphic designer or corporate branding expert.
3. How will it look in greytones?
Sometimes your logo might be printed on a black-and-white printer or copied on a copier, and you should consider how your logo would come out then.
Especially on a website, many make the mistake of letting the logo take too much space. The website is about your visitors, not you. They're not really interested in seeing your huge logo on each page. Your web pages are prime real estate to tell your visitors information that they might be interested in.
A logo for use on a website needs a resolution of 72dpi, but if you send that to a printer, they will moan about it. For good printing quality you should have 300dpi or more. So be sure you use the right resolution for the right purpose.
6. Image type
Logos should be created with vector based graphics. Why? Because with vector based graphics you can scale the image up and down without loosing image quality. If your image is made in rastergraphics - like a JPEG image for example - you will loose quality if you blow it up, for instance to put in on a billboard. A good program to create vector based graphics is Adobe Illustrator. From the master file you can then produce files in various sizes according to your need.
7. Inclusion of text and details
Including names, addresses, URL's and other things into the logo picture is not such a good idea in my opinion. This is because these details can change, and if they do, you don't have to recreate your logo. Use text, slogans, URL's and so on separate from the logo picture itself.
Having a transparent background for the logo can be quite useful. Transparency means that you can place the image on a non-white background, and it will blend in. If it has a white background, you can't place it on a non-white background without it becoming a bit of an eyesore. The GIF and PNG file formats support transparency, JPEG does not. PNG is now supported almost everywhere, and is a good format to use for logos on the web.