It's worse that slightly outdated. It's difficult to read and follow even for people, like myself, who have been a Christian their entire lives. I can't imagine the effort that an uninitiated person would have to put into reading it.
Here's something I just pulled out at random from the KJV, it's the first chapter of Hebews....
Hebrews 1:1 God, who at sundry times and in divers manners spake in time past unto the fathers by the prophets,
2 Hath in these last days spoken unto us by his Son, whom he hath appointed heir of all things, by whom also he made the worlds;
3 Who being the brightness of his glory, and the express image of his person, and upholding all things by the word of his power, when he had by himself purged our sins, sat down on the right hand of the Majesty on high:
4 Being made so much better than the angels, as he hath by inheritance obtained a more excellent name than they.
5 For unto which of the angels said he at any time, Thou art my Son, this day have I begotten thee? And again, I will be to him a Father, and he shall be to me a Son?
6 And again, when he bringeth in the firstbegotten into the world, he saith, And let all the angels of God worship him.
7 And of the angels he saith, Who maketh his angels spirits, and his ministers a flame of fire.
8 But unto the Son he saith, Thy throne, O God, is for ever and ever: a sceptre of righteousness is the sceptre of thy kingdom.
9 Thou hast loved righteousness, and hated iniquity; therefore God, even thy God, hath anointed thee with the oil of gladness above thy fellows.
10 And, Thou, Lord, in the beginning hast laid the foundation of the earth; and the heavens are the works of thine hands:
11 They shall perish; but thou remainest; and they all shall wax old as doth a garment;
12 And as a vesture shalt thou fold them up, and they shall be changed: but thou art the same, and thy years shall not fail.
13 But to which of the angels said he at any time, Sit on my right hand, until I make thine enemies thy footstool?
14 Are they not all ministering spirits, sent forth to minister for them who shall be heirs of salvation?
Now compare that with the NKJV...
Hebrew1:1 God, who at various times and in various ways spoke in time past to the fathers by the prophets, 2 has in these last days spoken to us by His Son, whom He has appointed heir of all things, through whom also He made the worlds; 3 who being the brightness of His glory and the express image of His person, and upholding all things by the word of His power, when He had by Himself purged our sins, sat down at the right hand of the Majesty on high, 4 having become so much better than the angels, as He has by inheritance obtained a more excellent name than they.
5 For to which of the angels did He ever say:
“You are My Son,
Today I have begotten You”?
And again:
“I will be to Him a Father,
And He shall be to Me a Son”?
6 But when He again brings the firstborn into the world, He says:
“Let all the angels of God worship Him.”
7 And of the angels He says:
“Who makes His angels spirits
And His ministers a flame of fire.”
8 But to the Son He says:
“Your throne, O God, is forever and ever;
A scepter of righteousness is the scepter of Your kingdom.
9 You have loved righteousness and hated lawlessness;
Therefore God, Your God, has anointed You
With the oil of gladness more than Your companions.”
10 And:
“You, Lord, in the beginning laid the foundation of the earth,
And the heavens are the work of Your hands.
11 They will perish, but You remain;
And they will all grow old like a garment;
12 Like a cloak You will fold them up,
And they will be changed.
But You are the same,
And Your years will not fail.”
13 But to which of the angels has He ever said:
“Sit at My right hand,
Till I make Your enemies Your footstool”?
14 Are they not all ministering spirits sent forth to minister for those who will inherit salvation?
There are surely passages that would give even a more stark difference but that one will do. It isn't just the antiquated words and phrasing but also the fact that the KJV doesn't use paragraphs but instead starts each verse on a new line which makes it not only harder to actually read but makes the thoughts harder to follow.
It's antiquated. It's usuable in a pinch but there isn't hardly a modern translation that isn't easier to read and understand for normal daily usage by the normal Christian.