A man asked the Prophet (sal Allahu alaihi wa sallam), "What Islamic traits are the best?" The Prophet (sal Allahu alaihi wa sallam) replied, "Feed the people, and greet those whom you know and those whom you do not know." [Bukhari]
One of the minor signs of the Day of Judgement is that people will only give salaam to the people that they know. [Ahmad] Nowadays we see that salaam is not considered the right of every Muslim. It used to be that salaam was given to any Muslim, whether one knew them or not. In fact, if you offer salaams to strangers today they might look at you funny, trying to figure out if they know you. Nevertheless, let us make an effort to revive this sunnah for the love of the Prophet (sal Allahu alaihi wa sallam) and as a practice of one of the best traits of Islam.