To address more specifically Minimus' question, I'd first say that the replacement of the tetragrammaton (YHWH) by "Lord" is not universal: the Spanish Reina-Valera Bible, or the Portuguese Ferreira de Almeida, which are still used by most pentecostals ans evangelicals throughout Latin America, regularly have an equivalent of "Jehovah" in the Old Testament (as has been said, there is no YHWH in any of the thousands of manuscripts of the New Testament, and on this point the NWT is clearly cheating).
Substituting "Lord" to YHWH was a Jewish practice in pre-christian times, either in the Hebrew reading "Adonaï" or "Elohim", or in the Greek Septuagint "translation" by "kurios" or "theos". The NT texts imply that, as is clear in Paul's reasoning on "kurios"-Lord, applied to Jesus, in Romans 10 for instance (the reasoning is lost when putting "Jehovah" in verse 13, as does the NWT).
My guess is that the "taboo" on pronouncing Yhwh in late judaism is a direct consequence of the rise of monotheism. Israel's god naturally had a name when he was one of the many gods in a polytheistic view (see for instance Judges 11:24, where Yhwh is the name of Israel's god as Kamosh is the name of Moab's god). When this god became God (starting from the Exile in Babylon, and specifically Isaiah 40--55), he didn't need a name anymore, and the traditional name was either theologized (as in Exodus 3) or deemed too sacred to be articulated...