Counterarguments to "KJV-only"? Some people believe the King James Version of the Bible is the only version English-speakers should be using, as it is the only inspired translation.  (This is referred to as &quot;KJV-onlyism.&quot;)
Basically, the KJV-only position is that:

The Bible is God's word

God promises to preserve His word

E.G., Psalm 12:7 (read this article)


This preservation is accomplished via a perfect translation (one per language)

The KJV is His perfect, preserved translation in the English language

The reason other English Bibles differ from the KJV is because those other Bibles have been tainted by sin and error


What are the main counterarguments to this view?
I am specifically looking for answers which would be convincing to an English-speaking Protestant with a high view of Scripture.
Some major problems with "KJV-onlyism" lies in the assumptions it makes and some of which you enumerated.

From an evangelical perspective, we accept the idea that the Bible is, indeed, the word of God.  Specifically, "men spoke from God as they were carried along by the Holy Spirit."


  20 knowing this first of all, that no prophecy of Scripture comes from
  someone's own interpretation. 21 For no prophecy was ever produced by
  the will of man, but men spoke from God as they were carried along by
  the Holy Spirit.  2 Peter 1:20-21 ESV
  
  20 Knowing this first, that no prophecy of the Scripture is of any
  priuate Interpretation: 21 For the prophecie came not in olde time by
  the will of man: but holy men of God spake as they were moued by the
  holy Ghost.  2 Peter 1:20-21 KJV 1611


Preservation via a perfect translation

The problem starts with the third point you enumerated, specifically that the preservation is via a perfect translation, that is one for every language.  

Why does the preservation have to be in a translation?  Why can't the preservation be in the original languages and those manuscripts?  How could we know that the KJV is the perfect translation into English?  The KJV was certainly not even the first translation into English, as it was preceded by the Wycliffe, Tyndale, Bishop and Douay-Rheims translations, among others.

Does it follow that the first translation into any language is a perfect translation and should never be updated, no matter how much the language changes?

In fact, the KJV has been changed quite a few times since it's original release.  No one reads out of the 1611 edition anymore, as our language has changed significantly since that time.

Also, the whole idea that there must be a preservation in each language at all times misses the point that most languages didn't have any translation until recently, and there are many that still don't.

So, this assumption has a hard time fitting reality and even logic.

Sin and error in other English translations

The assumption that only one translation can be accurate in any language for all times leads the KJV-only people to assert that any other translation into English must be somehow imperfect, and many even assert that sin is the reason.  The translators of the KJV must have been the holiest of men, and all other people who ever attempt to translate Greek and Hebrew into English today must somehow be sinful.  

That doesn't even make sense.  The English language is different today than it was 400 years ago.  Even if there were a perfect translation at that time, we don't speak that language anymore.

Also, why must experts in Hebrew and Greek today who translate the Scriptures into English be somehow tainted by sin?  Is it not possible that they could also be men and women who fear God as much or more as the translators of the KJV?  There are certainly men and women today who are translating the Scriptures into new languages.  Is it holy and righteous to translate into new languages, but the same people who translate into English somehow introduce sin and error?

Conclusion

The KJV-only teaching is rife with assumptions that are just illogical.  There certainly are poor translations into English.  Translation is not an exact science all the time.  Conveying meaning of one language into another is sometimes quite difficult.  The KJV is an excellent translation, among several others.  If you lived in the 1600's, there was nothing as good as that, even if you wanted to read the Apocrypha (which was in the initial printing).  Today, the language of the KJV is no longer spoken.  The NASB and ESV are probably two of the very best of the bunch, in my opinion.  I also use the NIV. 

There are quite a few other allegations from the KJV only crowd.  It could be interesting to address each of these in separate question.  I know James Melton has a booklet entitled "How I know the KJV is the Word of God", which has some statements that are absolutely ridiculous in it.