Sheikh Nadwi mentions several key characteristics that marked the short Ummayid rule of the Muslim world. Those include the return of the tribalism mentality and the idea of the superiority of one race over another (particularly Arabs over non-Arabs), which the Prophet of Allah (peace be upon him) had worked hard to eradicate. Also, there was a very dangerous change in the drivers and motivators of people, for many no longer performed good deeds (fighting in battle, generosity to neighbours,...) to get closer to Allah (swt), but rather to be talked about amongst the people.
There was also the unjustness of the rulers who would take from the poor and spend on their lavish lifestyles. Thus came to existance a very pampered 'aristocratic' class of people who were preoccupied by worldly pleasures. Nevertheless, this did not affect the majority of the ummah who still had a great respect for scholars and rightoeous people.
This is the world into which Umar bin Abdul Aziz was born and in which he was raised. It was said that his perfume could be smelt everywhere he went and that he had a particularly elegant walk which some of the women would try to learn. That was before his ascension to the 'throne' of khilafah.
The first thing he did was replace the unjust rulers of the Muslim provinces. He stopped the opressive taxes and was strict about rulers not accepting gifts or bribes, and about them not spending lavishly of the people's wealth.
On a personal level, he returned all the gifts he got to beit al-mal (the treasury of the ummah)and gave of his wealth and his wife's jewlery in charity. His zuhd and aversion of worldly comforts and pleasure was beyond that of any of the worshippers, let alone kings. He was also very conscious about not spending any of the ummah's wealth on himself. He would put off a candle which was fueled by money from the treasury if one busied him asking about himself (Umar's wellbeing).
He opened his doors to the people and announced rewards to those who would bring him news of the true state of society, or would advise him on something of benefit to the ummah.
He was able to embody the role of a true Muslim khalifah, one who takes responsiblity of the administrative and ruling affairs of the state, yet is also responsible for the knowledge and righteousness of the people (a role which had become restricted to the scholars of the time). He would send letters to the rulers of the Muslim provinces reminding them of their duties, guiding them to the straight path, enjoining the good and forbidding the evil.
Afterall, Umar bin Abdul Aziz was a knowledgable scholar of his own right. Mujahid (a famous scholar of the time) had said of him:"we came to him to teach him, but we did not leave until we learnt from him".
One of the greatest favors of this revivor on the Muslim ummah is his attention to the Hadith of the Prophet of Allah (peace be upon him). For although hadith was written as early as the time of the Prophet (peace be upon him), it was Omar bin Abdel Aziz who started the movement of collecting and preserving it by focusing the scholars' attention to it, just as his great grandfather Omar bin Al Khattab had encouraged the first khalifah Abu Baker al-Sideeq to do with the Quran (for more on Hadith collection, check out this post).
The reforms which Omar bin Abdel Aziz began were not favoured by all, and some worried they would lead to financial crises and losses for the ummah. However, what happened was to the contrary. The Muslim ummah was blessed with prosperity and security to an extent that the rich did not know where to spend their zakat. Yahya ibn Saeed narrates that Umar bin Abdul Aziz sent him to collect zakat from Africa. He did so, but could not find any poor people deserving of the money so he used it to free slaves.
The great khalifah passed away in 101 A.H., only two years and five months from the day he took responsibility of leading the Muslim ummah. May Allah (swt) have mercy on him and reward him for his great deeds.