Running a multilingual site can be a real pain, depending on what you exactly want to obtain. Drupal comes with an internationalization module that does a real good job on most cases. However, mine didn't fit within its features. Some of my content is written in English, other in French — exclusively. There is no use to translate every page to both languages.
What bothers me more in the current i18n module is that it always prefixes all your local URLs with the current language. That is, instead of having '/my/node', you'd have '/en/my/node' or '/fr/my/node'. This is not really serious by itself, but I started worrying when I realized that pages without any translation were both available under '/en/' and '/fr/', and that the only difference between the two versions was the language of Drupal's user interface. Having the same content referenced under different URLs is not a clean design to me, and it becomes objectively very bad when it comes to Search Engine Optimization.