it's a bunch of things.
The fast ramp up likely meant things machinery/processes never got fixed. So some legitimately terrible stuff got released. Like my car was delivered with a bumper that wasn't aligned on one side and was protruding by a few mm's. Tesla likely knew about this before shipping it, but opted to take the chance on me keeping the car. I did take delivery with the promise that they'd fix it. They mostly fixed it, but they likely lost me as a long term customer. I'll probably jump to Porsche once the Taycan is released. It's not just this kind of issue, but all around the 3 just isn't that durable/rugged. I want something a bit more well put together. I do like the car, but it's mostly because I really enjoy EV drive trains.
With the Model 3 the price range is enormous so it's hard to the perfection you expect from an $80K car on vehicle that is priced at $35K in a different configuration. If you were to take complaints I think most of the people complaining about panel gaps, and misalignment are going to be from P3D+ buyers like myself.
The paint sucks. There is no getting away from this one. I've bought a lot of cars in my life, and no one has paint as bad as Tesla. Luckily there are ways to fix this like ceramic coating, wraps, etc.
Mostly it comes down to lack of competition. You, and I both signed on the dotted lines. It's not like we had anything else we could have got that brought to the table what the Model 3 did. So we accepted it faults.
The problem is that other manufacturers do have similar price spread in models and have pretty consistent tolerances and quality control between all of them.
BMW for example builds, at the entry level, base model 320i for $35,000 and then at the high end $65,000 fully loaded 340xi is built in the same factory. Panel gaps, paint quality, interior assembly are all the same within that range. If assembly and finish are perfected for cars at the start of the range then the top end where customers are more demanding takes care of itself.
My car was not nearly as bad as some of the ones here. It does have a badly fitted glass roof and I suspect that's a source for some wind noise I am hearing on the high way as you can see that one leading corner of the roof is several mm higher than the one on the other side. I have several body panels misaligned that would not be noticeable to a less picky person than myself, but then again I've been driving premium German sedans now for almost 20 years before switching to Tesla.
My wife was getting out of the car yesterday and asked me if the seal around the B pillar mirror pod was supposed to look like it has a big rip in it... I checked the driver side, which is the same, and said "yup, that's how it's supposed to be".
When I was polishing the car prior to ceramic coating I ran into multiple fish eyes but fortunately no big scratches through clear coat, no major dust nibs (some small ones though), and no paint runs. My car isn't painted in the door jambs and I see it every time I am getting into or out of the car.
I'd say that I'm far more irritated with things going on inside of the cabin than I am with the annoying paint/finish quality of the exterior. Shortly after my car was delivered it started making a bizarre squealing sound when turning... technician thought it was a bad hub but they confessed to actually finding something loose inside the body of the car from when it was built. I had a rattling seat belt tensioner in the passenger side anchor point that was muffled by a mobile tech using some felt. I have some other rattles that seem to come and go. It's probably not worth the effort to try and pinpoint them, even though they drive me nuts, because Tesla have demonstrated to me that they don't have the tools or skills to fix these issues, and not sure they care.
I had some noise in my 328xi and they used a microphone setup to pinpoint it to a squeak in the center console, and they swapped it out.
After owning my 340xi for a year it started developing a squeak in the door seals after the car got wet that lasted for several days. The dealer replaced the door seals and when the problem came back they took the car for an entire day and used 3m tape on the entire edge between the doors and frame and eliminated the issue. They were always extremely responsive to my concerns about these issues.
Tesla technology is absolutely the best for electric automotive drivetrain.... but when other manufacturers catch up in this area and deliver a top level of fit and finish and a massive service network things might get a bit harder for Tesla... competition at that point will have arrived for them.
Today, there's nothing on the market that drives like a Tesla Model 3.