Some people think they can just slam out a few hundred words on what they think of a videogame and share their impressions with the world. They are wrong. Reviews - professional or otherwise - exist to be judged by fans of the series, and people must prove their worth to the faithful.
So there you are, surfing the internet and minding your own business when you see somebody has written something criticizing a game you love. Now, you're a busy fan. You've got things to do and you'd probably rather just spend your time enjoying the game than arguing about it, but you have a certain obligation to set things right. Specifically, this person is obviously an idiot and you shouldn't move on without letting them know.
But where to start? People who refuse to love a game as awesome as yours are staggering morons and nine out of ten thoughts that escape their malfunctioning brains are likely filled with confusion and error. Straightening out someone like that could be a full-time job, so be judicious with your arguments. Try to enforce these simple rules:
You're not allowed to review games you know you won't like.
Why review a game if it's not for you? If you're not going to like a game, then don't play it. Leave the reviews to the true fans.
You're not allowed to have an opinion on a game until you've played it.
It's unfair to judge a game before it comes out, or to decide you don't want to get a game when you haven't tried it. For a reviewer, this is incredibly unprofessional. Reviewers that do this are just attention-starved hacks.
Nobody should say anything negative about a game until they've played it themselves and have formed an opinion. Until then, they should only be allowed to say nice things.
You're not allowed to judge games by their stories.
Remember that it's not a plot hole if you can come up with a justification for it later. If you can, it just means the story is "ambiguous" or "thought provoking". It also means you're smarter than the hater. Remind him of this.
Besides, games aren't about story. Their stories don't need to be good. If you want story, go read a book. Games are about gameplay.
You're not allowed to judge games by their single-player gameplay.
When haters complain that the gameplay is poor, all they're really saying is that they suck at the game. They're terrible at playing and probably died a thousand times in the tutorial. Maybe they just suck too bad to see how awesome the game is? It's the job of the player to make the game fun, not the game designer.