I'm sure you've all seen it before: an original game that you love is granted a sequel that feels like the developers never really even played the first game. Many external pressures during a game's development can cause sequels to be vastly different from the original source material, like what the competition is doing, advances in experimental technology (Wii U, I'm looking at you), and most frustratingly, trying to appeal to a larger audience. The need for publishers to always sell more and more copies of a game (the essence of business) can have a massively negative impact on the qualities of a game that made it so unique in the first place, and result in a product that feels uninspired by the original source material.
The Deus Ex Franchise: Edit
A great example of this is with the Deus Ex franchise. The first Deus Ex presents a large number of socially imperative themes, such as corporate influence, loss of privacy, shadow governments, and the military industrial complex. In contrast, Deus Ex Human Revolution explores far less important themes, and bases almost all of its story around the idea of natural humans revolting against the idea of cyborgs - a conflict that isn't even well justified within the game.More than the story, Human Revolution also doesn't abide by the concept of technological evolution in its games - having more impressive technology in a game set twenty years prior to its predecessor just because it looks cool and is convenient. Whereas the original Deus Ex was set in 2052, where technology and societal themes were rooted in modern-day realistic concepts, the more modern Deus Ex games (prequels beginning back with the release of Deus Ex Invisible War) are much more science fantasy and rooted in their own contained society that's not based on realism or contemporary social issues. The lack of these political ideals that made the first game so compelling makes the new games accessible to a larger audience - an audience that more enjoys being entertained with flashing lights and brutal action rather than social issues and complex thought. Which is fine if that's the way you like to de-stress after a long week - it's mine too - but the diversity of video games is lessened each time a game undergoes this sort of transformation. The Deus Ex franchise now, having strayed far from its predecessors, is more synonymous with actions cyborgs rather than societal and political consciousness.
For more on the details of this Deus Ex trend, check out Ross Scott's "Game Dungeon," a series where he comically evaluates old, and often times unknown-games.
Pokemon GO Edit
Pokemon GO also demonstrates this "straying from the source material" and "simplification of a sequel". While it's not the only Pokemon game in the series, and could be argued to be different enough than the other in the franchise that it shouldn't be considered a sequel, it was not pitched as being an entirely different game: it was pitched as being Pokemon.. but in real life.
The problem with this is that the game does not play ANYTHING like any other Pokemon game that emphasizes the catching and training of Pokemon. (That is to say, it doesn't play in a similar manner to the original games, their direct sequels, or the slightly related sequels like Stadium, Colosseum, or XD. The Mystery Dungeon series is advertised and plays in a much different manner.) Instead of your standard battle mechanics most hardcore Pokemon fans are familiar with there's a generic "Attack" and a "Special" attack. Gone are experience points, effort values, and instead you feed your Pokemon candy... LOTS OF CANDY..
All in all, Pokemon GO breaks away from the traditional Pokemon capture / train schema in favor of a more dumbed down and simple format that is unacceptable for at least one long-time fan.
Dragon Age 2 Edit
Dragon Age Origins was an exceptionally awesome game. It was successful in multiple aspects. However, Dragon Age 2 was just so bad. Repetitive side quests, unbalanced classes and skills, and the biggest problem is the awful level design. Players have to go to the same maps over and over again doing the same things over and over again. It ruined its predecessor's reputation. Luckily, Dragon Age Inquisition came out the save the franchise.
Call Of Duty (anything after Modern Warfare 2...probably)
Producers of call of duty, as its done by numerous, very quickly lost its path. I'm going to speak to the play experience when discussing the series. Very shorty after the first Modern Warfare the Call of Duty series began replaying the same missions with different skins. The story lost ingenuity and missions were replayed with "new" characters. Most likely due to a focus of online play. Somehow they have managed to sell millions of copies over the years of the same game with a different title. Ghost is an example of making a huge selling point of accurate fish AI, really??