For me, it's the concern that newer translations are very likely derived from older translations; even when they attempt to translate the original Hebrew/Greek, they'll tend to be colored by other translations, be they other English translations or the Latin and German translations preceding the first English translations.

That being said, I usually go with Young's Literal Translation due to its explicit focus on, well, being a literal translation of the original Textus Receptus and Masoretic Text; whereas even KJV does its share of editorializing, YLT actively avoids it. Unfortunately, the consequence of such a literal translation is that it makes even KJV look modern and comprehensible by comparison ;)