When you look at the history of manufacturing, there are three distinct stages:
The first stage was the age of the marketeers: A very monolithic manufacturing process which made products sub-optimally, but the lack of quality was compensated by the power of marketing... The Ad-men sold whatever you could produce and you produced whatever your factory could make with the resources available to you... It wasn't great, but it was better than nothing.
That stage evolved into the age of the engineer: now manufacturing got optimized and you started to offer a product of better quality and optimized the resources. This led to a higher quality of product, but now rather than the marketeers, the engineers told the customers what they needed to buy... The perfect example is the ultra-thin watch: at one stage, the Swiss watchmakers started a headlong race to make the thinnest mechanical watch to compete with the quartz watches. Everybody competed to make the thinnest watch possible, but nobody listened to the customer's needs: which was not thinner, but more functionality (complications) to compete with the quartz watches. Maybe you can see the same thing happening in Ultra-books, where people are making thinner and smaller laptops, but out-compromising the functionality that discerns the Ultra-book from the Tablets.
Now comes the third age: the age of te designers. When additive manufacturing delivers on all the goods, the only major constraint will be the design of the product, not the resources, not the fixed set-up costs, not the production efficiency. This will mean that you will actually be able to efficiently produce what the customer wants. This does not mean that marketeers and engineers will become obsolete overnight, but the importance of design is going to become larger as technology progresses: right now a designer may design a very cool car, but the engineering filter will look at production efficiency and remove design elements and then marketeers will smooth out the non-conformist elements to make it sellable and you end up with just another car. Now designers will be at the consumer end of the production line, rather than the far side.
There are of course some human limitations to this scenario, which mainly has to do with attention-span and time. Humans only have so much time and energy and would only spend it on things that they need to survive or are passionate about. This is the very reason why "interactive" TV never worked: you do not want to control the outcome of TV, you just want to sit and watch... So people may not want to design their own doorknobs, but they would want to design their own wedding rings (at least for their first marriage).
the second human limitation will be ability: some things you can design yourself on a piece of napkin, but when it comes to making your own car, you would not want to sit on top of the fuel tank... Some products will need expert guidance. Some of this expertise will be stored in design software parameters that will make sure you can only design within safe boundaries, but some designs will need human expert checks.