Three factors to consider here:
1) TSLA is the most shorted public company. That means there are a tremendous number of people, and more importantly, institutional investors, that have heavily bet on its demise. When that happens, you will inevitably see negative media coverage, some more innocent than others. Yes, shorts can buy news.
2) I've been on a lot of car forums over many years. In general, you are going to see more of a concentration of negativity on them, generally because the people that are happy just go drive their car and don't make posts. I'm a good example. I'm a Model S owner, but these days I visit here because I'm interested in the Model 3 ramp. I visit the Model S forum every once in awhile, but usually only because some quirk or problem happens and I want to see if there's a good answer. So already you can expect to have a generally negative slant, where a lot of posts are "this happened, what do I do?"
3) This one is not confined to cars. It has to do with the psychology that goes along with the waiting process. The saying that "waiting is the hardest part" is pretty true. When there is a highly desirable product, whether it be a car, a phone, software, etc., people that are going through the waiting process tend to have bouts of negativity. I have no doubt many reservation holders have over-saturated themselves with the details of the car they haven't even got to order yet and they are going stir crazy! Enthusiasm and expectation can quickly turn into impatience and (hopefully temporary) disappointment.
Long story short, every Model 3 owner I've talked with is absolutely delighted with the product. I hope as production continues to ramp up that only continues.
I've had my share of small issues with my '14 S60, but this car has completely changed my outlook on personal transportation. It's so fundamental that it's hard to communicate to folks that still want to make comparisons to cars you have to drag to the gas station once every week or two. I get that everyone doesn't agree with that, but the good news is once you're in one, you don't need to argue with anyone, you have the luxury of driving it and enjoying it every day, and not thinking about the negativity anymore.
Super tl;dr
