Agrarian-based society; 95% of the population depended on the land, and natural disasters meant that the land couldn't supply enough food - this led to starvation and famine.
The Manchu dynasty itself lacked a strong central government, and didn't have support from all provinces. It allowed corruption and relied on the support of the aristocracy.
The Dynasty lost face to the foreigners because of giving concessions, land and other resources to them. Also, because the foreigners had "extraterritoriality", or were immune to Chinese law, the indigenous population looked and felt inferior.
Rebellions were not adequately suppressed, and this resulted in more money being paid out by the government... Reforms were slow coming, and in fact did not arrive before 1900. When they did come they were unsuccessful anyway - there was too little too late.
The Dynasty was forced to allow three successive thrones be passed on to the Empress Dowager, because there were three child emperors (1861, 1875 and 1908). This woman's attitude played a big part in the Dynasty's downfall.
Opium was not banned, and if it was (locally) then restrictions were not adequately put in force. This resulted in millions of bits of silver going out of the country to the traders, and the population's health going downwards.
Scholars and students sent abroard were the elite - they had a high rank in society and they came back with many new and different ideas and disrupted society.