Articles from Divine Love Ministries:

This parable contains one of the most radical teachings of Jesus to his disciples. The scene of this parable is the Parousia yet its central message is not so much about "the last judgment" as it is about the "core of the moral teachings" of Jesus which should be observed here and now.
The faithful Jew expects himself to be judged on two standards - the observance or non observance of the law. He also expects a preferential treatment for being a Jew. Now listening to this new standard which says that everyone (both Jews and Gentiles) shall be judged based on their reaction to the needs of others, posed a great challenge to them. 
All nations ... here refer to all people of every race, language and culture
By insisting that the final judgment shall be based on our actions does not in any way imply that faith in God is not necessary for salvation. For Matthew, faith in Jesus is the first movement of a person toward God. Faith in God must necessarily be translated into action. James 2: 20
Separate sheep from goats... In Palestine, it was common to see sheep and goat feeding together, yet at the end of the day each go to their separate places of rest.
Comparing the good people to sheep - an animal with a low IQ and often gets into trouble, points to how infinitesimal our good deeds are when compared to the generosity of God.
The King - Jesus is addressed as king in this parable. This title is not very common in the gospels and probably points to its ecclesiastical expansion.
The kingdom -  this is does not necessarily refer to the kingdom that Jesus proclaimed but to the "eschatological kingdom" that is prepared "from the foundation of the world." But the message proclaimed is relevant to both notions of the kingdom. (The rabbinical tradition taught that the kingdom of the Messiah was created before the foundation of the world).
The corporal and spiritual works of mercy - ministry to the basic needs of one's fellow human being forms the list of needs mentioned in this parable. Judgment is primarily focused on our behavior toward our fellow human being. These are simple works which we are not accustomed to see as our duties towards others.
"...and did not minister to you?" Jesus identifies himself with those to whom service is given or denied. Our behavior toward our fellow human being is inseparable from our behavior toward God (love of God and neighbor). The surprise of those who are condemned can be understood because they never imagined that God could be encountered in their fellow human being.
Eschatology implies that every person is capable of a final decision that gives one’s life a permanent character. Both the righteous and the wicked make decisions that are irrevocable and binding.
Lesson
1--Simple goodness - each of us can achieve greatness in life by doing simple things in great ways. We often wonder what we can do to help others in need. We may not be able to build a school for the education of children, hospital for the sick, house for widows/orphans, or prevent natural disasters, yet each of us is capable of feeding the hungry, clothing the naked, visiting the sick, etc. (Mk 9: 41, Mtt 10:42). It is simple goodness that pleases the Master and wins His approval.
2--Unconscious goodness - the people who were praised in the parable did not actually know the full implication of their good deeds. They were unconscious of their charitable works - their corporal work of mercy; their kindness, sympathy and generosity were quite spontaneous. This is the type of goodness that pleases the Lord. Your right hand should not know what your left hand is doing so that your father who sees in secret will reward you (Mtt 6:3). The best kind of giving is when the giver does not know to whom s/he is giving and the receiver does not know from whom s/he is receiving. Good deeds done for the sake of reward or prestige are not godlike. Real goodness does not want its name mentioned. Its thrill is the thrill of seeing someone go home happier than one was earlier.
3--Pride and regret - "if we had known that it was you who was in trouble we would have helped quickly and eagerly enough." There can be an element of snobbery in giving. We would often give cheerfully to a well-known person who happened to be in need, while neglecting or would not notice the need of an ordinary person in our midst. True Christian charity should be free from selectivity or preferential treatment. We ought to give because it is good to help anyone in need.
4--Calculated charity - we do not want our charitable acts to be indiscriminate. Many would like to give to organizations that would manage their donations well while ignoring individuals that they do not trust. The only problem with this mind-set is that it removes any personal element from our charitable acts. How often have we donated to Red Cross and Salvation Army while ignoring the beggar on our streets? As Christians we must risk being swindled in order to help someone who is in need of our help. The calculated generosity is unchristian, for God did not measure His love according to what we deserve. Christ died for the sinner even more than He died for the good ones.
5--The principle of Judgment - God may not subject us to the test by asking us to recite the creed or to quote certain passages of Scripture or how sound is our theological perspective. The simple question is – WHAT DID YOU DO TO MAKE LIFE EASIER FOR OTHERS? The answer to this question is neither based on the great contributions to human welfare which the newspapers reported nor on orthodoxy, but on our actions and interactions with the people we meet every day. After her death, it was written on a certain mother's tomb thus "She made life easier" - an excellent tribute both in God's and human eyes. Our parable does not ask WHAT DO YOU KNOW? WHAT DO YOU THINK? All it asks is WHAT DO YOU DO? 
6--Whatever you do to the least of my ... Jesus taught us that God is our father. Therefore, whether we know/believe it or not, we are all children of God (brothers and sisters in Christ). The best way to please a parent is to do good for his children, for parents are more grateful for whatever good done to their children than if it is done to them. And so is God. In the strict sense of it, WE CAN DO NOTHING FOR GOD AND CAN GIVE NOTHING TO GOD, for God being God has everything. But we can do something for someone who is a son or daughter of God, hence the best way to do things for God is by doing them for our fellow human beings. WHATEVER IS DONE FOR OTHERS IS DONE FOR GOD.
It is love that determines whether people are good or bad -  if our love is active, failure to reach perfect morality in other ways will be rare, and it will be forgiven. But there is no substitute for love that is active and generous.
An old legend of St. Martin of Tours -  the Soldier Saint. One cold winter day Martin was entering a city and a beggar asked alms. Martin had nothing to give him. But the beggar was blue with cold and Martin took off the old battered soldier's cloak he wore, cut it in two and gave half to the beggar, who blessed him and was gone. That night Martin had a dream. In it he saw heaven and all the hosts of heaven and Jesus, in the heavenly places, was wearing the half of a Roman soldier's cloak. One of the angels came to him and said, "Master, why are you wearing that old cloak?" And Jesus answered, "My servant Martin gave it to me." The traditional enumeration of the corporal works of mercy is as follows:
To feed the hungry;
To give drink to the thirsty;
To clothe the naked;
To shelter the homeless;
To visit the sick;
To ransom the captive;
To bury the dead.
The spiritual works of mercy are:
To instruct the ignorant;
To counsel the doubtful;
To admonish sinners;
To bear wrongs patiently;
To forgive offences willingly;
To comfort the afflicted;
To pray for the living and the dead.