If anything has hurt modern video game design over the past several years, it has been the rise of 'freemium'. It seems that it is rare to see a top app or game in the app stores that has a business model that is something other than the 'free-to-play with in-app purchases' model. It has been used as an excuse to make lazy, poorly designed games that are predicated on taking advantage of psychological triggers in its players, and will have negative long term consequences for the video game industry if kept unchecked.
Many freemium games are designed around the idea of conditioning players to become addicted to playing the game. Many game designers want their games to be heavily played, but in this case the freemium games are designed to trigger a 'reward' state in the player's brain in order to keep the player playing (and ultimately entice the user to make in-app purchases to continue playing). This type of conditioning is often referred to as a 'Skinner box', named after the psychologist that created laboratory boxes used to perform behavioral experiments on animals. The folks that create the Penny Arcade - Extra Credits series of videos have a good video describing the use of the Skinner box in video games that is recommended viewing on this topic. The use of this type of design is at best ethically questionable and at worst deplorable. It is one thing to create a game that is so fun that players don't want to stop playing, and another thing altogether to create a game that preys on its players using behavioral tricks.
Freemium games are often examples of some of the laziest and most poorly designed games. They usually include some sort of 'energy meter' that is designed to limit how much time the player can play the game before they either have to wait long periods of time until the meter recharges, or pay to continue playing immediately. This often results in nonsensical gameplay elements such as the player's character getting 'fatigued' without any in-game recourse other than plunking down cash to instantly recuperate. Similar to this design is in 'endless runner' games where the player can 'revive' their character after colliding with an obstacle (which would ordinarily end the run). Another troublesome freemium design is to encourage users to 'pay to win'. The player can simply purchase the most powerful weapon/item/whatever in the game rather than earning it via playing the game. This has implications on game balance in competitive games, and raises questions about whether players are actually playing a game if they can just convert money into victories. In all of these cases, the game is designed to keep the player from playing the game unless they open their wallets.
Freemium is currently having noticeable negative effects on the video game industry and will have negative effects on the industry in the future. Perhaps the biggest impact is that freemium skews the market's perspective on pricing. Users are becoming accustomed to app and game prices that are unsustainably low. It's depressing to see complaints about games being 'too expensive' when they are priced at five dollars or less. (Michael Jurewitz, by the way, has an excellent series of blog posts on app pricing and why low prices aren't the only way or even the best way to achieve profits.) Zynga's rise to prominence was partially based on its use of freemium design, and its fall can be blamed in part on an over-reliance on freemium. It simply isn't sustainable to have such low prices, and the result is that legitimate game dev shops go out of business while the sleazy game dev shops create games that aren't really games but instead elaborate psychological traps.
Folks, games should not be designed by spreadsheets. You can do your part to support good game design by paying for games that are truly fun. Don't get trapped in the Skinner box.