The main risks of unregistered trademarks are: that you infringe other registered trademark, that your mark will be stolen and that it is unregistrable.
1. Thousands of trademarks are stolen or lost every year. If your trademark isn't registered, then it can be hijacked in some countries. First to use countries offer some level of protection for non-registered trademarks.
2. The main risk is that your trademark is similar with previously registered trademarks. In this case you'll infringe these trademarks, which might provoke the owners to sue you for trademark infringement. This is potentially dangerous and we recommend ordering a trademark search first.
3. You can be using an unregistrable trademark which is generic, insulting or descriptive and cannot be registered or protected. Not all trademarks can be registered, see this article.
4. These trademark are difficult to defend. Someone can start using a similar trademark and it will be extremely complicated to stop him.
5. Unregistered trademarks cannot be sold separately from the business in which they are used, unlike registered trademarks.
Questions about trademarks?
International Trademark Search
Check if your trademark is available for registration.
International Trademark Registration
Protecting your trademark is the most important investment you can make.
International Trademark Watch
Be informed of recently applied similar or identical trademarks.