I don't read the Bible as often as I should, but when I do, it strikes me that some of the word choices are pretty bland. The words chosen by the translators seem to completely miss certain nuances in the underlying language. Now, linguistically speaking, "perfect" translation is an impossibility. Most words in a given language will not have an exact analog in a different language that perfectly captures all possible shades of meaning. Moreover, Bible translators are also tasked with creating a text that is readable by audiences of varying abilities and vocabulary levels. I'm still not convinced these explain the blandness, though.
Consider the Greek word akatharsia. It appears ten times in the Received Text, on which the King James translation was based. I reference the Received Text here purely because there's a handy online tool for exploring the Greek.
The King James translates akatharsia as "uncleanness". Modern translations like the NIV, NASB, ESV and HCSB choose the word "impurity" in verses like Eph. 5:3 and "uncleanness" elsewhere. Below are the verses where akatharsia appears. Reading them all together, one begins to get a feeling for what this word actually "means", and why various choices of English words may or may not be appropriate.
Woe to you, scribes and Pharisees, hypocrites! For you are like whitewashed tombs, which outwardly appear beautiful, but within are full of dead people’s bones and all uncleanness.
Therefore God gave them up in the lusts of their hearts to impurity, to the dishonoring of their bodies among themselves...
I am speaking in human terms, because of your natural limitations. For just as you once presented your members as slaves to impurity and to lawlessness leading to more lawlessness, so now present your members as slaves to righteousness leading to sanctification.
I fear that when I come again my God may humble me before you, and I may have to mourn over many of those who sinned earlier and have not repented of the impurity, sexual immorality, and sensuality that they have practiced.
2 Cor 12:21
Now the works of the flesh are evident: sexual immorality, impurity, sensuality...
They have become callous and have given themselves up to sensuality, greedy to practice every kind of impurity."
But sexual immorality and all impurity or covetousness must not even be named among you, as is proper among saints.
Put to death therefore what is earthly in you: sexual immorality, impurity, passion, evil desire, and covetousness, which is idolatry.
For our appeal does not spring from error or impurity or any attempt to deceive...
1 Th. 2:3
For God has not called us for impurity, but in holiness.
1 Th. 4:7
Given these, we can draw some conclusions about what this word might have "meant" to a first century Greek-speaking audience. Mat. 23:27 ties akatharsia to the sort of uncleanness associated with decay and decomposition. Most everyone views decomposing bodies as disgusting and unclean, but for a Jew, this type of uncleanness would have had an additional ceremonial dimension. Rom 6:19 positions akatharsia as the opposite of dikaiosynē, or righteousness. 1 Th. 4:7 corroborates this meaning by contrasting akatharsia with hagiasmos, or holiness. Now consider the typical English translations: "impurity" and "uncleanness".
These were undoubtedly chosen because they best represent the completely literal meaning of akatharsia, which comes from the root word katharos meaning "pure" or "clean". We get the English word "catharsis" from the same root. The problem here is that "impurity" and "uncleanness" do not necessarily carry a negative connotation in English, and from these verses it seems clear that akatharsia was meant to carry a negative connotation. For example, one could discuss the level of impurity in a chemical solution, and it's not necessarily a bad thing. These words also fail to capture the "decay" aspect present in Mat. 23:27 and the spiritual dimension present in Rom. 6:19 and 1 Th. 4:7. Let me suggest two possible alternatives:
1. Corruption. The most common meaning of corruption centers around political dishonesty. We complain about "corrupt" officials. However, it can also be used to describe physical decay. Consider these definitions from the Random House Unabridged Dictionary: "moral perversion; depravity", "perversion of integrity" and "putrefactive decay; rottenness".
Corruption would seem to be a perfect fit for Mat. 23:27, since it captures the connotation of putrefactive decay. It would also be suitable for Eph. 5:3 and some of the others. Alternately one could use:
2. Defilement. This word is rarely used in English, and carries with it an explicit religious connotation. "To defile" is the exact opposite of "to consecrate". Random House gives these definitions for defile: "to make foul, dirty, or unclean; pollute; taint; debase" and "to make impure for ceremonial use; desecrate". So "defilement" is essentially "impurity", but a special kind of impurity; one that is religious or sacramental. This translation is strongly supported by Rom. 6:19 and 1 Th. 4:7, and especially the latter. Hagiasmos, which the ESV translates "holiness", is also translated as "sanctification" other places in the KJV and ESV. If I had to pick an exact opposite for the word "sanctification" in English it would be "defilement".
So there you have it. If I were translating the Bible, suffice it to say the English I'd choose would be substantially richer and more descriptive than what's in today's most popular English translations. Then again, I haven't studied New Testament Greek, so there's a possibility that I'm just adding meaning where it doesn't exist.
P.S. The ESV's choice of the word "passion" in Col. 3:5 is also pretty terrible. "Passion" in English has an almost universally positive connotation. Contrast this with "lust", which is what the other translations use, and which seems vastly more appropriate in that context.