First and foremost - logo must be unique and easily recognizable. After all, he is the "face" of the company. Allows consumers and contractors to identify exactly your business. Simultaneously, he also performs the task of protecting your business from unfair competition. If logo, trademark properly registered, no one except the owner has no right to use them without risking prosecution.
Second - originality. Of course, we can afford once in a lifetime buy sneakers «Adibas», but even if the surrounding is not quite make out another letter in the title, it's like ordering at a restaurant sturgeon "second freshness.
Third - versatility. A good logo, as an elite cognac, age does not spoil, but forgive me, those who now enjoy re-branding. Of course, it is unlikely your logo will remain for centuries, but when you try to create just such a scenario. So try to explain intelligibly hired a designer: it works not only for you but also for posterity.
The fourth requirement (although it is more about the graphic part of a trademark or a trademark) - associatively. But here is not to overdo it. For example, if your company is engaged in construction or sale of heavy construction equipment, the trademark would not be very logical to look stylized flower. And another thing: do not forget to think about corporate colors, which will form the basis for the further development of your company's brand book.