Plants vs. Zombies 2 came out recently, and unlike its predecessor it's a free to play game. It has been released as an iOS exclusive instead of a $10 PC and Mac game, EA/PopCap have said that it will come to other platforms in the future. When this was first announced people were, to put it mildly, not happy with that bit of info. Here's the good news, the game doesn't attempt to massively gouge you and all of the levels can be played and unlocked without paying any money. On the flip side there have been additions and changes to the game design that almost certainly wouldn't have been made had this just been a buy to play game.
Slight side note, Plants vs. Zombies on iOS is a $1 game with in app purchases. It wasn't originally that price and the IAPs were added much later, I think around the same time as it dropped down to the $1 price. The transition to F2P makes more sense with that context in mind.
The original game was a simplified tower defence game, taking the elements of traditional tower defence and compressing them down to its barest essentials. Largely that remains intact in the sequel, you still use sunflowers to generate sun and you plant peashooters, wall-nuts and other plants to fight off the zombie hoard. The two main new features to this are plant food and special powers. Plant food is dropped by some zombies and can be used to give a one off boost to a specific plant, put it on a sunflower, it immediate drops lots of sun, peashooter and it'll do huge damage to its lane. The powers all use the touchscreen, but they are all variations of temporarily use gestures to kill lots of the zombies.
Both of these can be bought using coins, this used to be the only currency in PvZ and was used to purchase upgrades and unlock new plants, now these boosts are the only thing they are used for. This has two effects, one is that using one of the powers or buying plant food is the equivalent of a cheat, it breaks the balance of the level. The other is that it needlessly complicates the gameplay which was always supposed to be simple and elegant. Adding in these mechanics doesn't make the game more deep, just more complicated. The powers don't fit in as a core mechanic because it's gated by the use of coins and it doesn't feed into the core gameplay of balancing out the use of offensive plants and resource generating sunflowers. Plant food is dropped by certain zombies during a level but the way that you use it is either as a get out of jail free card in an emergency, or to just generate more sun. In both games you can buy coins using real money, all of the things that can be bought in PvZ2 with coins are single use items.
As in the original you unlock plants through playing through the campaign and you get a steady stream of new plants. Some plants are locked behind pay walls, most notably the Snow Pea and Torchwood. The Snow Pea was a staple of your defence in PvZ because it had the slow effect and single handedly made a lane much safer than it otherwise would've been. Torchwood was used to buff already existing defences with extra damage and the two were mutually exclusive in their usefulness. A flaming pea would render any slowed zombie back to full speed so you were forced to choose between the extra damage or the slow. Both are available in the store for two pounds fifty, each. The selection of plants you have available to you and the plants that you choose to take with you determines how you intend to play out the level. Having neither of the two more important plants from the previous game behind a paywall (they aren't currently available by any other means) is fairly limiting.
In the original there were five different arenas and they all followed a similar pattern, four standard levels, one mini-game, four more standard levels and then a boss level. In PvZ 2 this ls largely the same, except you now have an over world map. And a lot more mini-games. In the original these were always put outside the main game and while they are still somewhat off the main line of levels, they are much more prominently displayed as well as being significantly more valuable to progressing through the game. Calling it an overworld map is slightly misleading in that there isn't really any freedom in the map, what you have is a main path and then a series of upgrades/plants behind locked gates. These gates require keys to unlock, which are randomly dropped during play. Finishing the boss level on a world I had amassed all of 3 keys, which is just enough to unlock the cheapest door.
This wouldn't be a problem if it was only the mini-games behind those gates except many upgrades and plants are behind these gates. The original had some plants and upgrades be unlocked using coins, but in general it's much slower to get the keys to unlock all of the upgrades than it was in the original to buy them with coins.
The bigger problem with progression is that when you finish a boss world you can't immediately progress to the next world. Instead it gives you a giant gate with a Star unlock, the second world takes 15 Stars to unlock and 30 Stars are needed to unlock the third. Stars are also spent to unlock the next world so you'll need a total of 45 to unlock all the main levels are currently in the game. Getting Stars involves going back and replaying previous levels and you can only get one star per run through level. This essentially means that you're playing through the same levels over and over again just to get to the next bit of content. They try to change it up by adding challenges to the levels to get the stars, but it's been designed this way in order to slow down the speed at which you progress through the main content. If you just want to move on you can always just pay around four pounds to just unlock it immediately. This destroys the pacing of the game's progression, you go from unlocking new plants and new zombie types at a steady pace for 10 levels, to grinding out those same levels just to get to the next world.
Some of the upgrades have been moved to pay walls as well. There are duplicates of all the paid upgrades on the world map, but it all stacks. Buying an additional seed slot will boost your seed total to eight at max (eight was the max number of seeds in the original). New upgrades have been added to the game like getting refund sun when using the shovel, so you end up getting about the same number of upgrades as you did before. But having eight seed slots instead of six or seven meant you could carry much more variety into a level. It allowed you to carry more one of use plants like the cherry bomb, or maybe multiple plants that served the same purpose, like having both the Wall-nut and Tall-nut so that you have two different walls on cooldown.
Plants vs. Zombies 2 is not a bad game, by any stretch. Is it as good a game as the original? No, not at all. There's a lot of talk about how some games do the free to play model right and wrong and there have been many people that have said that PvZ 2 is an example of it being done right. I disagree with that assessment because they've changed the design of the game massively to accommodate that business model and as a result much of what made that game as great as it was has been eaten away at. Plants vs. Zombies 2 doesn't attempt to gouge money from you at every opportunity, but if that's the standard we're using for doing Free 2 Play right, then the Free 2 Play space is going to continue to be filled with games that don't respect players and their time.