1. He introduced "capitalist joining party" in the party constitution. Regardless its necessity, it does make many grassroot people feeling uneasy, how could a communist making wealth out of comrades? Why would a grassroot communist do selfishness work while his comrade can exploit him?
2. He was overtly "artistic" and "entertaining", showing off his musical and singing talents in front of foreign VIPs, especially Americans. Being entertaining is something that a Chinese leader should never do unless the audience is regarded as family or best friend. American politician is not. He made himself appear eagerly please the enemy, therefor weak.
Historical reference: 《
·廉颇蔺相如列传》, King of Qin tricked King of Zhao to play music for him, Lin Xiangru forced King of Qin to bit drum for King of Zhao in return.
3. He stayed on the post of Chairman of CMC for a year after handing over presidency to Hu Jintao. This makes Hu Jintao like a puppet. This made him like a "tai shang huang" and is bad in Chinese political norm. I know, Deng did that to Jiang as well, but that isn't an arrangement that people prefer. It is also different since Deng was one of the funders of PLA, if he didn't leave the Army in 1950s, he would have been a grand general. Jiang has no such history, giving him no reason to stay on.
Jiang's prolonged stay is also perceived by many to have weakened Hu's authority over other big power players such as Bo Xilai who dared to challenge the succession. The perception may be wrong, but it fits the negative view on "tai shang huang".
Historical reference: Qian Long emperor of Qing passed the throne to Jia Qing but kept many powers while his confident He Shen is eating the state inside out.
4. Jiang accepted interview by Mike Wallace during which Wallace referred to Jiang by the word dictator. Jiang is seen as appeasing and weak when he stayed on the interview and tried hard to defend him not being dictator. From Chinese stand point, such insult can only be met by forceful counter act. On top of that, many people are not happy that he made that interview in the first place. Mao Zedong accepted lots of foreign interviews, but never a person like Wallace who acted as a moral judge. Jiang just tried too hard to emulate Mao but doing it at wrong time, with wrong people and wrong tactic.
In contrast: he is expected to act like Deng Xiaoping treating Thatcher with more blunt and harsh words. If it was Mao or Deng, the answer would have been "I am proud of being the dictator serving the people".
I remember the TV program "Rome". When Brutus told Marc Antony "I have men of quality", Antony replied "I have the mob with me". That is the kind of proper answer to challenge from morally pretentious arrogant.
I must say that, all these things do not negate what he did right.