A Logo is Simple, Memorable and Relevant.
Having designed nearly 100 logos throughout my career, I've learned that while logos are important, they aren't nearly as important as the brand behind them. Without an emotional connection to the company or product behind a logo, logos are worthless. Building a brand is far more complex and involved than just designing a cool logo, but they do play a role in representing a company or brand and if done right, they become memorable road signs leading people back to your brand over and over.
There are three critical attributes that all great logos have in common. They are all simple, memorable, and meaningful.
Keeping a logo simple is the most important attribute of logo design and yet probably the hardest one to execute. Most companies and brands make the mistake of thinking their logo needs to tell the full story of their brand. So they add lots of detail and words and letters and icons and anything else they think will tell this big story. But in truth, a logo is simply an identifier - nothing more.
The biggest mistake people make in creating logos is looking at what their competition is doing and then copying them. This is the exact opposite of what you should do. Make it unique, different, odd or unexpected. Look at your competitors and then go in the complete other direction. Turn it upside down, turn it backwards, flip it over, underline it, circle it, cross it out, or erase half of it. Just be different.
Find the meaning in your brand and then find a way to convey that meaning and evoke an emotional response. Whether its a color, a drawing, or tagline, give it a meaningful connection.