Based on scriptural reference, some argue that shudrAs are born from the feet of brahmA ji, and hence they are lowest. If you want to understand the importance of feet, you should ask a person who is disabled and cannot walk. The correct way of understanding the symbolism is as follows:
brAhmanas are depicted as born from the head of brahmA ji, symbolizing that they are the teachers and preservers of knowledge, as knowledge is stored in the head.
kshatriyas are depicted as born from the chest of brahmA ji, because the chest is a symbol of virility, thus symbolizing their strength and their status as protectors of society.
vaishyas are said to be born from the stomach of brahmA ji, because it is through their very economic activity that society can feed itself, and also due to the fact that farmers, who are producers of food, are vaishyAs.
Lastly, shudrAs are said to be born from the feet of brahmA ji, because the feet represent labour. It is the feet that take us around from place to place, and shudrAs represent this section of society. There is nothing unholy about feet; after all, it is the feet that bears the weight of the body. The shudras with their labor are the ones who take us on a path of progress .
A Brahman is a scientist who does fundamental research and shudra is some one who uses that fundamental research as an application, the vaishya commercializes the application and Kshatriya makes sure that application does not harm the society. So all four are needed equally and deserve equal respect.
With regards to gunas, all four Varnas carry all three gunas in varying proportions. Predominance of tamasic guna in shudra means their primary occupation involves routine work, they are not predisposed to experimentation. Their work is unskilled or semi skilled. They are happy with stable and secure life and are not ambitious. And predominance of satvic in Brahmans means their predisposed to creativity and research. Such people do not have materialistic goals. Kshatriya predominantly rajasic with higher satvic and lesser tamasic so they are devoted to actions for public service like administration and protection but seek power as a reward. Vaishya is rajasic with higher tamasic and lower satvic so they are entrepreneurs with profit motive. I.e. More selfish than Kshatriya.