Include a basic summary of what the game is about and how it’s played.
Contextualize how you played it, so people can get an idea of your process and extrapolate how that may have shaped your review. (in playing BOTW, I made it clear that I aimed to play the game a certain way)
Make clear observations that attempt to explain how things work in a nuts-and-bolts way (pointing out velocity, acceleration, state, etc), instead of unclear descriptive words (smooth, slippery, tight, etc). Build up to a conclusion, don’t start with one and forget to justify it.
Never mark a game down for being hard. You’re allowed to say it’s too hard for you, or one part is too hard relative to others, but difficulty isn’t inherently bad. Difficulty affects depth, which is more important. Does the way the difficulty was implemented create more or less depth? (more by encouraging you to try different things instead of stick to one thing, or less by requiring a specific solution) Remember, there is no such thing as artificial difficulty, it’s hard or it isn’t.
It is however legitimate to mark a game down for not being clear about how it works. A game has to provide at least enough information for you to figure out how something works, by at least providing you with enough knowledge to experiment, and feedback to learn from your experiments. It’s okay for a game to not explain everything, but there should be enough information provided for you to figure things out. Double checking beginner playthroughs (lets play culture has made these common) can be informative about what players commonly get confused by.
It’s also legitimate to mark a game down for being unfair, such as not informing you something is the wrong choice before penalizing you.
Always finish the game, unless you have a really good practical excuse. It’s not technically necessary, because of the next guideline, but it should be a point of professional pride. 100% completion is optional, but if you do go for it, don’t review under the assumption everyone will go for it, and try to retain a perspective of what the game is like without going for 100%.
Moments that are temporarily really good, or really bad shouldn’t be counted towards the overall conclusion/score. A game should be judged by what the majority of the content is, not one really deep moment, or one really shallow moment. So I won’t judge Nier more positively for having deep gameplay during the intro, or Dark Souls negatively for Lost Izalith existing.
Double check how other people play the game (especially skilled players) to make sure you’re not missing something important about how the game works. In single player games, it can make sense to incorporate high level techniques that are logical extensions of the core gameplay (like DMC combos), but not ones that aren’t really representative of the core game. In multiplayer, anything goes.
Don’t play on the easiest difficulty (unless that’s your thing and you make it clear to your audience)
For a single player game, try to target the experience that you consider most representative of the average player’s experience (see the 100% remark). For a multiplayer game, try to target the reality of what competition is like.
Non-conventional reviews can explain how a game might be fun in a way that not everyone may experience, which can be valuable to people, but they also limit their impact.
If a core mechanic inconveniences you, ask yourself why it’s there and what it accomplishes before writing it off. Similarly, if something about a game seems unfair, do some experiments with it and double check if there was legitimately nothing you could do about it. Don’t blame the game for your problems. Again, check if other people are having a hard time with what you’re struggling with or if there’s a solution you overlooked.
Avoid jargon (unless it saves you a lot of words overall, then define it the first time you use it. If you only use Jargon terms once in the review, ditch them)
Describe the thing you’re talking about as directly and clearly as you can. Try to keep your wordcount low, sentences short, and syllable count per-word low. There are online tools for this, called the flesch-kincaid test. When I hear big technical words, I immediately think the other person is trying to show off how smart they are at the expense of writing something easily understood.
Don’t use cliche language or phrases. Avoid using buzzwords that seem meaningful, but conceal actual meaning. (eg. it’s “immersive” “responsive” “feels X” “makes you feel like you’re X”)
Don’t bother talking about price, framerate, load times, bugs, or Resolution. The best games of all time have fluctuated on all of these things. Unless one of these things is so bad that it actively and frequently harms your ability to play.
Maybe talk about how the monetization model harms the game.
Don’t bother with the history of the game or developers. There’s a space for that in broader discourse about a game, but it (usually) doesn’t belong in a review. Try to focus on the game itself, unless outside context really does help explain the game itself.
Your play time with a game doesn’t give you more or less authority with a game, it’s about the quality of your observations. Don’t feel pressured to finish a game fast or slow.
Try to be thorough, try not to include unimportant details.
Try to sample many of the different tools and playstyles the game makes available to you. If something is cheesy and too good, it deserves to be called out, but also try playing without it. In multiplayer, no mercy. Players will often take the path of least resistance, so effective but simple & boring strategies will ruin a game for most players.
If you’re reviewing an arcade game, no continues allowed (if an arcade game literally cannot be beaten without continues, that’s a mark against it). If the game has save states, try to limit yourself to either autosaves, or saving only at the start of levels to avoid completely destroying the difficulty.
If you dislike a game’s genre, you still need to criticize it on grounds that fans of that genre would agree with, not for simply being in that genre.