A city cannot be completely autonomous, it depends on it's countryside for food, water and every other resource. The only thing a city produces is services or turning one resource into another (ie manufacturing). Without the country to provide resources, a city can't exist.

So the smallest level of complete autonomy you can achieve is either a nation with substantial land area, or a very small "city" that is closer to a farming village.

"Working with something they love" is not an incentive. Maybe for you it is, but the vast majority will work to provide for themselves and their family, which by definition is greed.

They'll work the minimum to receive their food and other allocations, then work elsewhere (in the black market) to better their families living standard above and beyond. This is competition, the secondary work market and the desire to provide the best for our families (ie greed) is what generates it. Competition would simply move into the black market.

Administering this utopia would be a logistical nightmare, without prices shortages and surpluses would go unchecked until it is too late. With food that spoils or things to take a long time to make (such as mining or doctors), these things are hard to predict or spot even with price signals (such as with the surplus of oil and lawyers today), without it it'll be impossible right up to the point a crisis begins.

There would be a drastic reduction in living standards. We are used to goods from China, oil from Middle-East, internet from the US, cutting all these off and regrowing local replacements would be much more expensive and as a result, there'd simply be less things. Very few would accept a reduction in living standards for themselves and their family, making this change impossible in a democratic way.

Innovation and creativity would slow, as the lack of competition removes this drive. Appointing people's jobs as 'innovation creators' is a solution, but still inferior to having an entire nation thinking.

And then there's competition from outside the utopia. Unless you managed to convert the entire world, the progress of the rest of the world would eventually fuel greed without your utopia, causing defections and eventual collapse. Communism regimes of the past are examples of this.