What are common criticisms against using the KJV? 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.