Many of the changes were bug fixes from early production e.g. poor panel alignment and other various build quality issues. Some changes were due to customer demand e.g. flat folding middle seats. Some changes were performance enhancing e.g. upgraded battery and power train specs. Also many software updates along the way, both introducing new features and improving many existing features. It is concerning that they had so many quality issues to start off with, but encouraging that the vast majority of those issues have since been addressed. A more conservative approach would be to develop the car more extensively before initial release, but this is literally the cutting edge of automotive technology and many of the issues were ultimately quite trivial (although very annoying for typical OCD owners used to quality at this price point - or at least the perception of quality through brand recognition).
I think there is still a risk in buying a Tesla, but life's getting too short for me to worry too much these days! I'm not buying shares in the company, only a specific product commitment for a few years. I noticed quite a few Tesla owners buy again after their first encounter, so can't be all that bad. On the forums, most owners think the overall experience is worth any "early-adopter" hassles they get. There are very few complaints about how the cars actually perform.