Of course "nice" is a bit of a vague word in contemporary English. It means everything from minute, as in precise measurements, to subtle to delicately perceptive to carefully neat to pleasing and agreeable, with a few more things in between. If we decide that logically you probably mean "pleasing" by the word "nice," we can isolate a few of the pleasing concepts Rudyard Kipling puts forth in "If."
In Stanza one, Kipling addresses the important concept of self-trust, especially when other people have lost trust in you and doubt you. Kipling adds meaningfully to that popular notion--used by arrogant people as well as sincere people--by admonishing that we "make allowance for their doubting too;". This has a double meaning. It first means to take into serious consideration that the doubts might be founded in fact and, therefore, correct. It also means to be generously-minded to the doubter and be willing to overlook the offense.
Stanza two expresses the very pleasing and encouraging idea of "stooping" to reconstruct you life from the rubble after a physical or metaphorical breaking of things in your life, particularly your life's dream or work ("you gave your life to"), which puts me in mind of events like Katrina. Stanza three addresses the need to take risks in life, then admonishes to not complain or dwell on the loss should the risk prove to be an unsuccessful one.
Finally in the last stanza, in which Kipling talks about how to gain continued forward motion despite the harshness of forever-forward-marching time, he also admonishes virtue and humility. He says "if you talk with crowds," implying a very famous person who is sought after for speeches to large audiences. The admonition is that despite fame and greatness, virtues of goodness and upright living and kindness and justice must still be clung to. In addition, if you "walk with kings," your humility and compassion must be maintained.