It's easy to say "well if you don't like a game, don't buy it or play it". But that belies the fact that if said game that I didn't like or buy becomes a phenomenal success, I'll have to suffer through years of copies and clones trying to capitalize on the success of the game. This kind of thing is often referred to as "revolutionizing" a specific genre.
For instance when Unreal Tournament and Quake 3 Arena came to the market in 1999, they expanded and, in my mind, more or less perfected the competitive online FPS genre. UT Classic is still up there as one of my all time favorite games. This started something of a Golden Age of FPS games. It's hard to determine where it started exactly (for me it was UT Classic), but the FPS games released around 1999 changed the face of gaming forever. Unreal Tournament, Quake 3 Arena, Team Fortress Classic, and Counter Strike were all part of a "revolution" that made "fragging" your friends and making the "gibs" fly a part of all our lives.
Of course they were all heavily influenced by GoldenEye 007 and Turok: Dinosaur Hunter for the Nintendo 64 of all things (which released in 1997), but this was the first time you could do all the friend fragging online with a wider group of players. All of these games and their iconic combat style, weapons, and multiplayer dominance were successful because of players' opinions. If nobody reacted positively to GoldenEye 007, we likely wouldn't have any of the games that followed. Or, if we did, they would be completely different. Fortunately, this is an example of a "Revolution" I could get behind. I liked the game(s) everyone was trying to copy, so I had fun with each new game.
My point is that the entire industry is fueled by opinions, critical reactions, and word of mouth. That's not to say that whether a game is good or not will mean the success of the game. It just means that if people thought the game was good they'll tell their friends, who will probably buy it. If a popular review site (or combo review site like metacritic) has a positive review of a game, people will probably buy it. The game will make more money. Competing companies will see this trend and try to capitalize on the popularity of the title, creating very similar games. It's Economics 101.
So when you read my posts, someone's opinions on a forum, or hear someone ranting about how much they hate a mainstream title, understand that it's not as easy as "if you don't like it don't play it". Because their choices may be either play the game or one of its clones, or don't play a game in that genre for the next several years.
Hell there is an entire industry just of people's opinions on games. Magazines, websites, and TV shows that just focus on someone's opinion of a game are an industry unto themselves.
But don't take my word for it. Look at former editorial director for Gamespot Jeff Gerstmann, who was fired after posting a less than positive review for the game "Kane and Lynch: Dead Men". His firing was reportedly a result of pressure from Eidos Entertainment, the developers of Kane and Lynch, who had bought considerable advertising space on Gamespot. There was a veritable exodus of editors from Gamespot following Gerstmann's termination, citing that the site was caving to advertising pressure and moving to artificially improve a game's score based on how much money they were given by the developers.
Or perhaps look at the oft-overlooked yet brilliantly designed Painkiller. It was generally an obscure little game that no one paid much attention to, despite how great it was. That is, until 'Yahtzee' from The Escapists Zero Punctuation created a rare positive video review for the game. This one man's opinion on Painkiller increased sales on Amazon.com by 7,400% in a single night.
So Like it or not this is an industry shaped by opinions, both positive and negative. That's why we have console wars, flame fests, and fanboys. I encourage everyone to express their opinions about gaming, and not hide behind phrases like "well if you don't like it then don't buy it".
If you do nothing about a game you don't like, you have no one but yourself to blame when you're stuck playing Halo 13 on the Halo 360 while drinking Halo Dew, because there isn't anything else on the market.