Arabic is a very rich language, and words have many shades of meaning. Thus in many languages it often requires more wordiness to get the meaning across, which detracts from the beautiful simplicity of the Qur'anic message. Arabic idioms, and the weight of the words, are difficult to understand and translate. While Arabic is a living language, the classical Arabic of the Quran requires more study in order to fully appreciate and understand the depths of meaning.
Another concern is the translator's familiarity with the target language. Some translations are done by individuals who are very knowledgeable in Arabic, but they are not able to render the intended meaning because they are not familiar with the nuances of the other language.
In the end, the dilemma is to find a translation that is linguistically and spiritually as accurate as possible, yet is readable and flowing. In English, such a translation is difficult to find, though many have tried.