Sadie- Gaming is a fairly diverse field (more than someone from outside would expect). The issue is who are you asking about Zynga's quality of games? If you ask a console gamer, they will tell you the graphics are lousy, the game strategy and actions are simplistic and so on. If you compare Farmville with World of Warcraft, you can readily see they are totally different products with different markets. So what customers are you asking?
Even within casual games, which is the market space Zynga occupies, there are diverse games targeted to various ages and interests.
As others mentioned, Zynga is expert at metrics and at what could be considered a form of behavioral economics. How much reward to give people to get them to continue playing. Another factor that may be reflected in feedback you are seeing is that more recently there has been a fall off in people playing Zynga's games so you may see some dissatisfaction with their more recent games. It is not easy to keep up a steady stream of heavy hitter games. Entertainment styles come and go and it takes work to keep up and to keep game products fresh. Zynga has been highly successful in a difficult market, but they do need to work to keep ahead of competitors.
I also agree with Alexander's comment on Blizzard. They make some popular games that cross types of gamers. Hearthstone is a more recent one that is good.