The easiest recycling will be of items that are printed in Plastic Filament. You can pretty much just melt it down put it back into it's standard size and reuse it. However, it gets trickier when you want to start using everyday products like plastic water bottles, and other plastic recyclables.

Recycling metal can be tricky, because of the alloys in each metal, and different temperatures that certain metals melt at.

Then it gets to be really tricky trying to recycle ceramics, wood, etc.

One day we will be there, but I don't think it will be until 3D printing has fully taken off, and some large companies with plenty of money can start researching ways to make recyclable filament, and machines that can extract materials that can be used to print with.