I know that many people and churches prefer using translations other than the KJV. Why? What are the common criticisms against it?
The usual criticism is readability. The KJV was originally translated in the 1600's, and even though it's been updated several times since then, there are still words that have different meanings now than they did in the English of the time. The literary style is clearly less modern than more recent translations.
Another criticism is based on the translation sources. The New Testament portion of the KJV is based on the Textus Receptus (Received Text), which has been subject to some criticism. This has resulted in some minor textual differences that can be traced back to the different source manuscripts.
I hesitate to answer more completely because the choice of a Bible version is personal, and I want to avoid stating that any version/translation is superior to others. There are those that feel very strongly about the superiority of their own version (KJV-only and those strongly opposed.) The textual differences mentioned above tend to fuel such arguments, which in my opinion, tend to be counter-productive.
Another criticism is the distinction between a translation of the original texts, and a paraphrase. The various Bible translations fall somewhere on a scale between word by word equivolence and thought for thought equivalence or paraphrase. The KJV is closer to the word-for-word end than many modern counterparts.
The KJV-only groups tend to prefer a direct translation of the original because it is less subject to be influenced by the views of the translator. They argue that an interpreted version is suspect, because the result is influenced by the translator's understanding of the meaning of the text. On the other side, the argument goes that the "paraphrase" versions are written so that the meaning is clearly understood, and it's therefore "easier to understand" than the "archaic" KJV.
The biggest problem with the KJV is that no one is fluent in its language any more. It simply is not written in an English anyone speaks today.
When you read the KJV the problem isn't words you don't know - you can go look them up in a dictionary like you do any other words you haven't read before.
The problem isn't complicated sentences or strange word orders - you can spend the extra mental energy to parse it even if you're not reading smoothly.
The problem is that words change what they mean over time. You can learn what changes have occurred, and sometimes you'll realise that the words don't mean what they think you do because they don't make any sense in context. But often both meanings fit the context and unless someone has told you that a word used to mean something else you'll just automatically read it with the modern meaning. So the biggest problem of the KJV is the one you don't realise you have!
An example of this is the word 'conversation', which is found in Philippians 1:27 in the KJV. The modern meaning of 'conversation' fits the context perfectly, but it's not the right meaning. Every modern translation, including the NKJV, translates it as 'behaviour' or 'conduct' or something like that. Unless you already knew that 'conversation' used to mean something else you would never think that it did from reading it. Later in Philippians 3:20 the word 'conversation' is used to mean 'citizenship'. I don't know if it used to have two meanings or was just translated poorly, but again it's the same problem. That verse doesn't make quite as much sense, so you might guess that something weird was going on, but you might not.
For five more examples, try this quiz: Do you speak KJV?
If you're willing to check every word you read in the KJV against an Early Modern English dictionary, then go ahead and read it. But if you're aren't, I guarantee that there will be times when you understand it incorrectly without even realising. Please, read a Bible translation in a language you actually know!
What are common criticisms against using the KJV?
Some feel that the Textus Receptus (original Greek and Hebrew) from which the KJV was translated is an inferior collection superseded by the Alexandrian text which is a little older. Concerns regarding the Alexandrian text are that it omits the end of Mark and some other differences. Some consider the text superior as it has been dated earlier and is regarded as less likely to have been altered as Alexandria had a problem with allegorical-ism which would be unlikely to motivate intentional translation alterations. I do not find these arguments sufficiently compelling to abandon the KJV.
One group of criticisms can be made against the KJV as can be made against any translation, particularly “word for word” translations. There is information in a word in one language that cannot be brought over into a single word in another language. For example English is often bereft of verb tense information that is in words of other languages. For example, the English translation may say to do something, the Greek may say to do it as well as indicating that it should be done only once or continuously. The missing information can have a significant impact on correctly understanding what was written.
Some cling to the KJV as a defensive reaction to the publication of various “translations” that seem threatening to Biblical tradition. Readers Digest published a “condensed” Bible in what I assume was a sincere desire to make the Bible more accessible. This is slightly different than using simple language to help those less able to understand as it helps those more easily bored. Then there are translations like “gender neutral” translations that are more alterations to advance a particular ideology. Those who use the KJV to hold at bay the invasive and corrupting forces at loose in the world, may find that they hinder themselves from the use of translations that could open greater understanding to them.
The most significant charge that can be made against the KJV is that its language (Shakespearean) is difficult for many today to easily comprehend. In all fairness, even some of the ideas are as difficult to digest as the language. A reason for these difficulties is the conversion of the populace into entertainment consumers. There is no reason for people to want to put forth the effort to read arcane language when they have little desire to grapple with difficult ideas.
One has to consider the dynamic power of God through his word. We can all too often see the Bible as a textbook or instruction manual and forget that it is God who works through it.
Deuteronomy 8:3 And he humbled thee, and suffered thee to hunger, and fed thee with manna, which thou knewest not, neither did thy fathers know; that he might make thee know that man doth not live by bread only, but by every word that proceedeth out of the mouth of the LORD doth man live.
1 Corinthians 2:14 But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned.
Even though there is a constant and swelling assault on the Bible, we should remember that it is God’s word and he will protect it.
1 Peter 1:23-25 Being born again, not of corruptible seed, but of incorruptible, by the word of God, which liveth and abideth for ever. For all flesh is as grass, and all the glory of man as the flower of grass. The grass withereth, and the flower thereof falleth away: But the word of the Lord endureth for ever. And this is the word which by the gospel is preached unto you.
To answer your question directly, the most "common" criticisms are that (a) the form of English used is arcane and (b) the manuscripts upon which the text are based - especially the New Testament - are somehow inferior.
I am not sure either of these criticisms are completely warranted.
One advantage of Elizabethan English is that it retained a distinction between the second person singular (thou) and plural (you) and was therefore better able to mirror the underlying Greek text. In modern versions such as the NIV or NASB, this distinction is loss, so one does not know, for example, when Jesus is addressing an individual or a group.
Regarding the second criticism, antiquity of the manuscript is not really any indication of authenticity. It is a hypothesis that cannot be proved, since we no longer have (or maybe never did have) the "autographs". The oldest New Testament manuscript we have may be 100 years older than the next oldest, but the fact remains that whatever we do have was written centuries after Christ in any case.
Another criticism that is sometimes laid against the King James Version is that it contains errors. These include Luke 13:32, which states Go tell that fox ..., even though the Greek says "this fox" and not "that fox"; John 5:28, which is punctuated to reproduce a heretical reading posed by Paul of Samasota in the 3rd century and refuted by John Chrysostom in the 4th; John 3:3, wherein the Greek indicates that Jesus really says that Nicodemus must be born from above, but the translation says that he must be born again. These, however, are errors that also have propagated into more modern translations as well.