In general, Zynga games show a trend of taking control away from the player and setting things up in such a way that peer pressure works in their favor.
Zynga makes a lot of flash simulation games. There's Farmville of course, where you run a farm. There's Cafe World, where you run a restaurant. There's Treasure Isle, where you dig for buried treasure in a series of tropical islands. There's Frontierville, where you're trying to create civilization in the middle of the woods. And most recently, there's CityVille, where you build a city.
What all Zynga games have in common (aside from all being concepts stolen from other social game companies) is that they all start out reasonably fun, but then as the game progresses and new features develop, they reach an annoyance level that makes them unplayable.
I've discussed Farmville before, so let's start with Cafe World. Zynga combines the restaurant simulation concept of Restaurant City with the crop timer mechanic from Farm Town, to create a game where you cook various dishes of food on stoves, then transfer them to counters where your waitstaff serves it to your guests. Each dish has a different timer - for example, hamburgers are done after 5 minutes and must be moved to a counter within 10 minutes after you started them.
When the game first started, it was great fun to plan out an entire menu of dishes and juggle all the different timers so that everything would be served together. The music, graphics, and animations were pleasant, and (just like Restaurant City) you could spend a lot of time fiddling with your restaurant's layout and decor, and visit other people to see theirs.
Likewise, when Treasure Isle first started, it was a very relaxing game to play, with the tropical island setting, music, and sound effects. The graphics and animations were pleasant. In Treasure Isle, Zynga takes the core of Treasure Madness and adds in elements that worked well from Farmville and Cafe World, for example giving you an avatar and a personal island to decorate. You dig up islands, one plot at a time, and collect sets of treasures.
But then came all the "collect ten of five different items to build a super-item" goals and other baggage that requires you to have many friends willing to send daily gifts. Along with those came the barrage of popups that get in the way of playing the game. Limited time offer to buy some trinket to decorate with! Limited time offer to buy the premium currency at a discount! Play some other Zynga game for rewards in this one! Have you invited enough friends yet! Send them some gifts! You still need X amount of Y, beg your friends for them! And look, you're almost out of energy - pester your friends for that too! And so on. At least half a dozen of them will ambush you just for opening the game.
Frontierville started out with all the annoying popups already in place - like this was a feature rather than a bug. It also introduced quests. Now, instead of having a bunch of arbitrary goals tacked in, the goals became the main part of the game. Even so, Frontierville started out reasonable, despite the detractive qualities of the popups and the way loot drops land right in the way of whatever you most likely want to do next. The first few quests are internally consistent with the concept of a homesteader moving into the middle of the woods and setting up a house and home.
But then they become more and more arbitrary, and nearly everything becomes the "collect ten of five different items" type. Worse, they add themselves to your quest list whether you want them or not - and then say "new goals!" in a blinking arrow that blocks whatever's behind them and doesn't go away until you click. In short, Frontierville quests are set up to prey on the obsessive-compulsive who must have everything in a task list done before they can relax, even if the tasks are increasingly stupid and pointless, and largely consists of posting tons of wall spam because you can't actually play the game at all without gifts from friends.
And then we have Zynga's latest offering. CityVille takes all the annoying features of Frontierville and combines it with the look and feel of Social City. The quests have gone to a whole new level of annoying. Now, instead of being merely a guide for what to do next, you're basically being told what to do - what decorations to buy and place, what building to build, etc. And of course you'll need a ton of neighbors because prime parts of the game (including quests) are unplayable otherwise. The only good thing about it is that it doesn't lag anywhere near as bad as the other, older Zynga games yet, so if you're going to look, do it early.