The name of the 'rebellion' in Chinese is 太平天国运动 and it gives a little insight into what it was - a huge social movement against the Qing government and arguably more than just a simple rebellion. One of the main reasons why this civil war of sorts was so catastrophically disruptive to the Qing was scale: Hong Xiuquan's Taiping Tianguo basically ceded from the Imperial government and came extremely close to actually overthrowing the Manchus all together. From Tobie Meyer-Fong: "[The Rebellion] afflicted sixteen or seventeen of Qing China's twenty-four provinces." The center of the rebellion, in South/Central China near the Yangtze delta, was densely populated and contained a large concentration of China's people. In the final years of the rebellion, with particularly brutal fighting, western observers noted that millions of civilians were slain or died indirectly, that is not as a combat casualty but as a consequence of the fighting itself. By the mid-19th century, the Qing government was already on the decline and having a massive war on its hands meant that the central government was even less effective in governing. Famines, epidemics of disease and forced labor contributed to the deaths.^(1) Another point to note is that both the Qing and the Taiping governments were weak to the point where it was difficult for a decisive blow to be dealt. This wasn't a case of one side having a decisive advantage over the other like for instance the American civil war where the Union had an enormous industrial edge over the south. The weakness of governments on both sides helped to prolong the suffering and drag it out for over a decade.
To add to the terror, civilians were deliberately targeted during the war. There were wholesale massacres occurring against civilian populations and many first-hand accounts of survivors detail how the people of a city might be killed in droves by the Qing, and once the Qing forces were driven out, slaughtered by the Taiping, and so on and so forth. The World of a Tiny Insect is one such account, written by Zhang Daye who had witnessed the horror as a child while living in Zhejiang.
The second part of your question is several centuries past what I am most familiar with and I don't know enough to provide an answer. u/EnclavedMicrostate?
I have addressed the question of the rapid growth of the Taiping in the past here and that of the death toll in the first section of this post.
I'll say that I agree on the whole with the existing answer /u/beepybeetle, but I will elaborate on one or two points:
Whilst Tobie Meyer-Fong's statement that '[the war] afflicted sixteen or seventeen of Qing China's twenty-four provinces' gives a good idea of the geographical breadth of the conflict it isn't necessarily a useful way of understanding the casualties. This slightly sad map shows what I mean: The war was mainly being fought over five provinces – Hubei, Anhui, Jiangsu, Jiangxi and Zhejiang – whilst the other ten or eleven provinces were drawn in through relatively isolated Taiping campaigns. Guangxi, Guangdong and Hunan were passed through in the original revolt (and a lot of Taiping recruits were actually Hunanese); Guizhou and Sichuan (and possibly Yunnan) in an independent expedition from Shi Dakai; and Henan, Shandong, Shanxi and Zhili during the failed northern campaign of 1853/4. This was not a case of the war reaching out to all corners and affecting everyone equally, but rather largely a result of concentrated devastation in the Lower Yangtze provinces.
Additionally, with regard to the length of the war, I must disagree with the view expressed on the relative strength of the two governments. The Taiping were consistently beset by a divided leadership which never really resolved itself, whilst the Qing gradually cleaned their act up, devolving military authority to Zeng Guofan's clique, whilst also formalising its relations with the West by establishing the Zongli Yamen for diplomacy and integrating the largely Western-staffed Imperial Maritime Customs Service at Shanghai into the government. For sure, neither side appeared to have an innate material advantage (although Taiping territory was small it was easily defensible and densely populated) but at the very least that success in formal diplomacy gave the Qing a huge edge when it came to courting outside support. Obviously, the question of the degree and nature of the significance of the Western intervention has always been somewhat under dispute, but it was nonetheless a major feature of the conflict and to some extent must be accounted for in considering how the war turned out.
