What is the "Good News of the Kingdom"? Who is Preaching it? And What Does "For a Witness to all the Nations" Mean? (Mt. 24:14)

At Mt. 24:3, Jesus was approached by his disciples and was asked: "Tell us, When will these things be, and what will be the sign of your presence and of the conclusion of the system of things?" In response, Jesus did not provide a specific time, saying that “Concerning that day and hour nobody knows, neither the angels of the heavens nor the Son, but only the Father." (Mt. 24:36) However, while the exact time of the coming end of this “system of things” has not been revealed to us, “the sign” that Jesus gave would let us know when we are in the time period known as “the last days.” (2 Timothy 3:1)

Telling us what will mark the last days, Jesus went on to say: “Nation will rise against nation, and kingdom against kingdom; and there will be great earthquakes, and in one place after another pestilences and food shortages; and there will be fearful sights and from heaven great signs.” (Luke 21:10, 11) Jesus also said: “This good news of the kingdom will be preached in all the inhabited earth for a witness to all the nations; and then the end will come.”(Matthew 24:14)

What is the "Good News of the Kingdom" Described at Matt. 24:14?

The Bible mentions that the entire theme of Jesus’ preaching was God’s Kingdom. For instance, Jesus said,

"I must declare the good news of the kingdom of God, because for this I was sent forth.” (Luke 4:43)

Most are familiar with the model prayer. In it, Jesus taught us to pray for God's Kingdom to come:

“Our Father in the heavens, let your name be sanctified. Let your KINGDOM come. Let your will take place, as in heaven, ALSO UPON EARTH.” (Matthew 6:9-13) emphasis mine.

Daniel 2:44 says more about this kingdom:

“In the days of those kings the God of heaven will set up a kingdom that will never be brought to ruin. And the kingdom itself will not be passed on to any other people. It will crush and put an end to all these kingdoms, and it itself will stand to times indefinite.”

After that, mankind will be under just one government, God’s Kingdom. The Bible says that that Kingdom will do God’s will and bring great blessings such as removing Satan and his demons. (Revelation 20:1-3) The Bible also mentions that faithful humans will no longer get sick and die. Instead, under Kingdom rule they will be able to live forever. (Revelation 22:1-3) The earth will be made into a paradise.
Also see:
God’s Kingdom - What Is It? When Will It Come? (bh chap. 8 pp. 76-85; Watchtower Online Library)
Kingdom - Links to Information (Search For Bible Truths)

Who is Preaching the Gospel of God's Kingdom Today?

Some religions have acknowledged the Christian obligation to preach in public places and from house to house Acts 5:42; Matt. 10:11-14; Luke 9:6), although this is often left to a limited group of missionaries or clergy to fulfill... not every member.

However, every member who is identified as one of JEHOVAH'S WITNESSES heeds Jesus' instructions when he told his followers to "make disciples of people of all the nations," and when Jesus himself set the example by "journeying from city to city and from village to village, preaching and declaring the good news of the kingdom of God." (Matthew 28:19, 20; Luke 8:1)
Also see:
Ways They Use to Share the Good News (jt pp. 19-21; Watchtower Online Library)
Their Worldwide Organization and Work (jt pp. 25-26; Watchtower Online Library)

Will the Good News Have to be Directly Preached to Every Man, Woman and Child on Earth Before "The End Will Come"?

Notice the wording at Mt. 24:14,

"...will be preached in all the inhabited earth for a witness to all the nations..."

Jesus did not specifically say that every man, woman and child on earth will have the good news directly preached to in the last days. He said that the good news will be preached in all of the Earth for a witness to all of the nations. This means that every nation will have the good news preached in it to some extent.

Interestingly, as of the time of this writing, Jehovah's Witnesses are preaching this good news of the Kingdom in 236 lands worldwide.
http://www.jw.org/en/jehovahs-witnesses/