Dear Mubashir, this is exactly what zakat system is from what I could humbly understand from Quran. It’s sad that the West has taken up the golden principles of this great message without giving it any credit (Although their way can still have many flaws and susceptible to corruption as they have completely given the reign of the whole system in the hands of the individuals and totally separated Deen from individual and all governing systems).
Zakat is not an individual act as per Quran but a system that needs to be implemented and so the amount for which cannot be decided by the individuals.
Brother Javaid described everything about Zakat perfectly just that the matter of who gets to decides what the surplus is. I think as Zakat is for the community and by the community, the decision about how much would be more than enough at individual level also needs to be decided by community according to its collective needs. In other words the level of what is enough for a person and what is more than enough has to be a collective decision keeping in view the basic needs of the community or nation as a whole for food, shelter, clothing and health, not individual choice and so decided by the group of those chosen ones or volunteers which fulfill the criteria of representatives as per Quran. Also if a person works hard and earns more than he is supposed to give once decided then he has right to keep it for his children or give it away in the form of charity… whatever he wishes to do he can do.
This can only be done by the group of sincere people, people who are true and pure and understand that:
The ranks of all individuals and nations are determined solely by their deeds, and your Lord is not unaware of what they do. [Calling yourselves ‘believers’ will not help. 2:8]
...And that everything belongs to the One Sovereign only and we have to meet our Lord eventually and so they work for the collective benefit of the humanity. Described by Quran as:
They fulfill their vows and pledges and fear (and therefore, guard against) the times when chaos and corruption might sweep the society.
And they feed, for the love of nobility, the indigent, the orphan and the captive.
[They help establish the System where the first ones to receive the provisions of God are the ones who are needy, who have lost their jobs, whose lives have stalled, the widows, the orphans, the ones who feel left alone in the society, the captives and those who are stranded on earth for any reason]
I can think of this as the only reason Allah SWT left for humans to decide the exact amount. Practically it is the soundest way too as there always will be some bad fish in the pond as well as many non Muslims to whom we cannot deny any facility if they are living among us and who need to be part of our community.
The last thing I must emphasize is that zakat system cannot work alone, it has to be accompanied by SALAT system which should rather take precedence in order for Zakat system to work, the reason it is not mentioned without Salat, if I am not wrong, anywhere in Quran. Both the systems are complementary to each other.