I saw a very striking pshat from Reb Chaim Shmuelevitz that I want to share. He says that Chazal's expression, as if dead, means that the person lacks a what in a healthy human being would be a pathway to empathy and assistance to others. This is not to say they are incapable, it means that it is not as simple for them.
The poor refers to one who is so burdened with his need to make a living, to find what he needs, (whether because of actual or only perceived poverty- איזהו עשיר השמח בחלקו and vice versa, as Shlomo HaMelech says in Koheles 6:4, איש אשר יתן לו האלקים עשר ונכסים וכבוד ואיננו חסר לנפשו מכל אשר יתאוה, ולא ישליטנו האלהים לאכל ממנו-) that he doesn't have the time to think about the needs of other people, and certainly doesn't have the means of helping others. This is reminiscent of Maslow's hierarchy of needs.
The blind, because we all have experienced the effect of the powerful image. We were all fully aware, intellectually, of starvation in Biafra, but it was a terrible photograph in Life magazine that shocked millions into a full emotional awareness and sympathy.
A leper, or more accurately, a Metzora, because both the cause and the effect of Tzara'as is being outside the community. The causes of Tzaraas are all manifestations of a basic indifference to the suffering of others, and the punishment of Tzaraas is to be driven out of the community.
One who has no children, we know, may not sit on the Sanhedrin (Sanhedrin 36b, and Rambam 2 Sanhedrin 3 -
אין מעמידין בכל הסנהדרין לא זקן מופלג בשנים. ולא סריס מפני שיש בהן אכזריות. ולא מי שאין לו בנים כדי שיהא רחמן
Rashi explains זקן. ששכח כבר צער גדול בנים ואינו רחמני וכן סריס, A person who has not experienced, or who has forgotten, what it means to raise a difficult child and to love him despite the pain he causes, is missing something that teaches a person to be merciful. Having children enhances the ability to sympathize, and one who has no children has a challenge in achieving that degree of sympathy.
I want to stress that there are poor people, and blind people, and people who have no children, that are gedolim and saints of kindness. What Chazal mean is that while a normal and emotionally healthy Ben Yisrael has a natural rachmanus, and has to be a rasha to choose to harden his heart and ignore those that need his help, these four people might have not that natural and automatic reaction of rachamim.
It is also possible that Chazal are speaking metaphorically. There are people who can see, but they are blind when it comes to to the needs of others. There are some that are so busy accumulating things that they are totally uninterested in helping others to make it. There are those that simply never experienced what it means to have a child, which means that your heart is walking around outside of your body. And there is the Metzora, who exhibits every one of these traits. Such people may be healthy, and wealthy, and happy, but they are dead men walking.