3D printing is for everyone NOT!
I had a 3D printer for a few years now. It's a beauty, isn't it? :)
I love how you hear that 3D printing will change manufacturing because anyone will be able to make anything in the comfort of their home. Once you actually try that you understand how far that is from the truth. A friend said that it's like modern woodwork - you can make nice and useful things, but you still need a specific set of skills. So I decided to gather a list of issues that you can encounter while producing items with 3D printer.
You need to be able to model in general (i.e. use 3DsMax, Maya, Blender, Modo, Sketchup, etc). As an alternative there a bunch of places where you can buy/download premade models, but if that's all you need a printer for, then it's not very useful.
The next step in modeling is being able to make models that are 3D printer friendly, i.e. the proper surface of 3D object (i.e. no polygons inside, no one-sided polygons, etc). Depending on your slicing software that you're using to convert your model into a printable format, your slicing software might take care of a bunch of issues.
You want to make your model light and strong. In order to do that you need to have a rough understanding about the strength of the material, chamfers, supports, etc. You can find a good guide here
If you're lucky, you might be able to choose and afford better slicing software, but in my case, Da Vinci 3D printer is bound to crappy slicing software, which has a limited setting, poor UX and has plenty of bugs.
Printing small/thin parts (for example 1-2mm thick antenna sticking up), because it simply prints layers too fast and they do not cool enough.
In some case, slicing software can make mistakes while converting a model (due to problems in the model) and it won't tell you about that. And if you're not paying a lot of attention you will find that out only after printing the part, i.e. after spending N-hours for the printer to complete useless part :)
I mentioned slicing software several times. That's something you will have to learn too. For every print, you need to select settings that match your goal best, i.e. thickness, rafts, supports, etc. You need to understand the process of printing in order to select correct settings (for example: at what kind of angle I need supports and does my model have such angle anywhere).
When buying a 3D printer you can find a statement about layer size that printer can print (for example Da Vinci 3D printer can print 0.1, 0.2 or 0.3 mm layers). Unfortunately, that doesn't tell you anything about actual precision on the printer. I would say Da Vinci can make precision errors of at least 0.6mm (when printing with 0.2mm layers).
I had an issue where temperature in the room was too low (and I'm taking by 4 degrees lower than recommended) and I kept getting broken prints. The problem is that the print cools too fast and it bends and then printing head hits the model and everything gets messed up eventually.
Sometimes printable material doesn't stick to the printing platform (usually due to insufficient amount of glue that you put on the platform), that messes up your print, so I have a habit to check how print is going at least during first 5 minutes because if something goes wrong the printer doesn't stop - it keeps going until the print is "complete" and you have one big mess in your printing area :)
My printer had loose parts resulting in poor precision, so I had to disassemble it slightly and modify a bit :) For some reason, you think about 3D printer differently - it's a machine that you supposed to tinker and customize. I would never bother to do that to a microwave oven or a washing machine.
You have to calibrate your printer occasionally and my printer at some point just kept giving an error during that process. Eventually, after spending a couple of hours poking it with a multimeter and cleaning it, I found out that the calibration process was broken because the printing head had some plastic on it.
If you allow your printer to finish spool of plastic to the last bit, then a small piece of plastic left in the printing head and you can not replace a cartridge until your remove that piece, and you can not take it out without (slightly) disassembling the printing head.
Once you produce your 3D prints they do not look as smooth as other plastic things, so you need to postprocess it. Acetone, rasps and sanding paper will be your new friends :)
My point is that there are lots of these small issues that you might encounter and you will have to figure out how to solve them. The process is not always straightforward. It is different than operating things like a washing machine, which you simply expect to work if it doesn't - then it's broken, there is no grey area in between. Of course, eventually, 3D printers will become better, but now 3D printing is more like an art than engineering ;)