A freemium model that's almost invisible
Plants vs. Zombies 2 will be a free game in the App Store, and PopCap says you can complete the entire game without spending a penny, but about 5 percent of the content is hidden behind in-app purchases. As you swipe through the in-game almanac, you'll notice that a couple of the locked plants are for purchase only, while most are unlockable by finding the right location on the map. None of them is required to finish the game, but some plants and upgrades are enticing enough that I think people will buy some of them anyway.
Upgrades like the Power Moves I talked about above are mostly unlockable by playing as well. For example, you can unlock Sun Boost for an extra 25 sun power by playing the game, but you can purchase the Sun Bonus upgrade for an additional 25 sun power. The other upgrades are set up in the same way. So, really, the items and upgrades behind the pay wall are usually for a bonus on top of what you already get.
Minigames and earning stars
As you play Plants vs. Zombies 2, you'll get a little gameplay variation through minigames. These levels have specific requirements or force you to use only certain types of plants. In one level I played I had to choose from three playing cards to determine what my last plant type would be. In another, the game gave me different types of plants as they slowly moved up the screen on a conveyor belt. The minigames are great because they mix up the gameplay a bit and it's always fun to see what challenge they're going to throw at you.
You'll also need to earn stars in order to unlock new worlds. You can earn them by playing levels you've played before, but each star has a specific set of requirements in order to get a single star at the end. Challenges include things like never losing a lawnmower, or never allowing the zombies to cross a specific line in the gaming area. Some of the requirements can be really challenging to accomplish, so expect to play through some levels more than once.
One minor annoyance
It's fairly minor to me, but some players will probably be annoyed with one part of the freemium model. Some plants that were common in the original game are now for purchase only. Depending on your particular strategy, you might be sorry not to have the Snow Pea plant that shoots and slows enemies. This was a plant you unlocked early on in the original game, but it's not an option in Plants vs. Zombies 2 -- now you'll have to pay to unlock it. Again, there are only a couple of plants like this and you can beat the game without them, but it's still a bit annoying not to have a plant that was free in the original game.
Still, the freemium model here is one of least annoying I've seen, and Plants vs. Zombies 2 is a fantastic sequel to an already great title. With new plants, new worlds to explore, minigames, and new emergency powers, it will be easy to get sucked back in to the familiar and extremely addictive gameplay.