For their part, Zynga isn't likely going to be very good at defending itself. They've got a formula that's currently making them rich. They're going to be very slow to want to mess with the system. Just as IBM was out-maneuvered by the wily newcomer Microsoft, and later as the blundering Microsoft was outfoxed by many smaller contenders, being successful in a new and quickly changing market can be a curse. You need people that can innovate and adapt to stay ahead of the curve. And if Zynga employed those sorts of people they wouldn't have needed to swipe their game designs from other companies.
The Zynga challengers are coming. I'm sure right now there is a small team of people coding away and dreaming of taking a bite of that Zynga cash. Here are some things those challengers might try:
More Flexible Usage
Farmville is bad at letting the players enjoy the game at their own pace. Sometimes you don't have a mind to play, but if you don't log in your crops will wither. Sometimes you're in the mood to play some more, but there's nothing left to do until your crops are done growing. This is a nice mechanic for Zynga, since it encourages people to play on terms that benefit the publisher by obligating them to keep coming back for lots of short sessions. (So they can see lots of different ads.)
But anyone that discovers a game that's just as "fun" as Farmville (has the same action / reward cycle) but can be played in a more convenient and friendly way is going to be drawn to leave their farm behind.
More Appealing Presentation
Farmville looks bland and stale. The music is tedious and the sound effects are nothing special. Compare this to the stuff churned out by PopCap. The PopCap artists are masters of making alluring interfaces, engaging visuals, and catchy tunes.
I don't know if they pioneered it, but PopCap has been doing this for years in order to rope in gaming neophytes. You entice someone into playing a fun Flash-based game, but then offer them the chance to try a "standalone" version with some extra features. Suddenly you've got people downloading and buying games.
Note how all of these changes will gradually turn some people into more avid gamers. Sure, some Farmville players will click away on their virtual farms forever, but for those who really enjoy the action-accomplishment-reward cycle of videogames, they will naturally be open to new and more rewarding experiences. Once you have a group of people who understand the market and can make informed buying choices based on their personal preferences, you have gamers. And more gamers is good.
All that needs to happen is for someone to apply the Zynga social model to something more entertaining than Farmville. And if you can't make something more entertaining than Farmville, you probably shouldn't be trying to make videogames to begin with.