First of all, respect to PopCap games. Their games are paragons of accessibility meeting good design. I own three of their games and one of them I play on three different platforms. You gotta give it to them, they know how to make an attractive game.
Let's take a brief look at Peggle. Why does everyone love Peggle so much? Accessibility! Every single person who can operate a computer can play it. The only method of interaction is to click somewhere on the screen. We do that all the time! Even without instructions, the UI is presented in a way that after moving the mouse and clicking once or twice, you understand exactly how Peggle works. The other way accessibility comes into play is the software requirements. The game runs on platforms that millions upon millions of consumers own: any friggin' PC made in the last 5 years, the iPhone, Nintendo DS, and Xbox 360. Try and think of someone you know who doesn't own one of those platforms (good luck). Now think about how many folks you know who own more than one of them. What potential for customer base! When a product is so widely available and for the cost of a couple bucks, many hurdles are leapt on the path to success.
If accessibility gets PopCap half way, good design takes them to the finish line. All PopCap games I've played to date have sharp, identifiable, attractive artwork. Bjorn the Unicorn, the Bejeweled gems, the Sunflower plant, etc. The sound design is memorable to the point of internet meme. All the audio/visual design is flawless and I haven't even started on gameplay principles. Their games are scary addictive. Perfect implementation gradual difficulty slopes and "one more level" syndrome. Bookworm and Bejeweled can be played over and over ad nauseum while Peggle and Plants vs Zombies share a similar formula with specific level designs and elements of progression & challenge. The former tend to appeal better to casual gamers looking to relax while the latter two having stronger appeal to non-casuals looking for the satisfaction of skill development and identifiable level progress. To me, both styles have their time and place in my gaming diet.
Game developers looking to tap the vast customer base that PopCap has discovered absolutely need to study the PopCap library and understand what it means to be accessible. This brings me to my own game developer aspirations. Not only are PopCap games simple to program in terms of scope and code complexity, but they are fun, too! I have plans to try developing a few casual games while constantly keeping in mind the principles that make PopCap games fun. The first two ideas are based on the careers of my wife and best friend; a nuclear reactor and an electrical power substation. Ideally, with their help I can make some games that are fun, accessible and informative!