The fruit of the work done is what's valuable, thus is what the employer buys from the worker by paying wages, or a slave owner forcibly takes from a slave. This is why an employer is interested in technical improvements: they reduce the required amount of work per unit of the resulting valuables.
The tension between the worker and the employer is about the price paid for the fruits of the work. Definitely, with standardized industrial manufacturing, it's about the amount of time and work conditions, because a worker can't significantly influence the performance of the machines he's using, or could not at Marx's times. As the Japanese (e.g. Toyota) have shown, workers with right motivation and right education can influence performance of industrial manufacturing quite significantly, e.g. by spotting inefficiencies and by lowering the defect rate.
Work has positive utility when it creates something of value. Considering the money paid for it as coextensive with the work is a mistake because people can work to create value without getting paid, as sometimes happens to unfortunate people. It is no more reasonable to treat the work inputs into a good as having negative utility as it is to treat the money paid for it as having negative value.
But work also has utility for the person who pays the worker, and this is extremely different from what you've described.
This difference in the utility functions for each party (one wants to maximise the return on work and minimize the work; the other wants to minimize the work and the cost on it) is what sets up most of the "classical" marxist class tension. This is despite their agreement that the less paid work that needs to be done, the better.