Having done it myself, I find several big issues with Japanese -> English translation:

• Honorifics. Referring to someone with -kun or -chan or -sama or -bucho all have very different connotations. Sure, you could just include the honorifics as is but I don't think that many people unfamiliar with the Japanese language know what -kohai means.
• Culture: Even if the translation is perfect, characters may act in bizarre ways for Westerners. "He knows that guy is evil, so why doesn't he shoot him?" "Shooting people is very serious shit in Japan." "Well no one told me!"
• Puns: The big one. To give an English -> Japanese example, how do you translate the joke, "I no that!"? Joke being, the "no" implies that the speaker doesn't actually know what "that" is, with the "no" taking the place of "know" since they sound the same. "No" in Japan is "iie", and "know" is "shiru". They don't sound alike at all, so how do you do it? Japanese is filled with puns like this, and most of them are completely lost on Westerners.

That said, I do support translators for giving Westerners a variant of the Japanese version. But there's no mistake that a lot is lost.