There are a few solid rules about logo design which are often violated and are pretty much guaranteed to create problems.

1. The difference in sizes between the largest and smallest text shouldn't be much more than 2 to 1. Logos are used at many sizes, so any text needs to be readable when used small; on the other hand not look overly "horsey" at largest size usage.

2. Most logos should be wider than tall (emblems an exception). This fits with the way they are mostly used.

3. Logos should be designed in such a way that they will work in black and white or possibly in two colors (possibly with some adjustment from a full-color version).

4. Logos should be designed in such a way that they work at a great distance or at a glance, as well as close up.

5. Don't try to make a logo do too much work!