Before I even say anything about this I need to reiterate that a completely objective review is and will likely forever remain impossible: everyone can and does have differing tastes. Things that can seem like a grind to one player enthral another, while other things that get some gamers adrenaline pumping prove to be immensely frustrating to others.
That being said though, there are a fair number of things that you can say about any game regardless of whether it’s from your favourite or most loathed genre. These elements are part of what can help make even a controversial review stand up against the inevitable criticism that will be levelled against it: because all criticism does have at least a hint of legitimacy to it, even if only a tiny one in some cases.
Most obviously a genre needs to be established: a lot of games tend to dabble in more than one these days, but there always should be one main genre that can be pointed out, lest you be playing a very strange game indeed. It could be a first person shooter with a smattering of RPG elements - a combo which is popular these days - or an action game with a lot of world based exploration - games like Infamous, Prototype, and Assassin’s Creed fall into this one - I know it may seem obvious as to what genre a game is, and going out of your way to mention it might invoke a “Hey, I can read the back of the box myself, thanks” reaction from some people, but sometimes there are things that go somewhat unaccounted for. Maybe the game starts off as a straightforward shooter, but then gets those RPG elements introduced a quarter of the way in as a “surprise”. Some people simply won’t appreciate that kind of thing cropping up, because it’s sort of bait and switch. So letting them know really does them a favour while constituting perhaps minimal spoilers at best.
Another thing always worth mentioning is how long the game will last: this can take the form of two very different sum totals. The first is how long it will take to beat the game at a bare minimum, with the second being likely an estimate of just how long it would take to attempt to find every secret and complete every side quest. Again, this comes down to choice: some people just want to play a game through to the finale and don’t care about what they might overlook as long as the core experience within the game was enjoyable. For others it’s going to be a matter of hunting down absolutely everything to get that fabled 100% (or more) game completion. Both styles of play are valid, but someone might be unwilling to spend 60 hours just to be able to finish a game, while others might think that a game that can be beaten with 100% completion in 30 hours doesn’t provide enough bang for their buck.
Other things that can be looked at from a mostly objective standpoint include graphics - they don’t have to be bleeding edge, but do they stack up well to the current industry norms, exceed them, or not meet them at all? - Of course if the graphics are meant to be stylized that needs to be taken into account: games like Katamari Damacy and Minecraft hardly have graphics that will blow out processors, but the way they are presented constitutes part of their charm. It’s the same thing with cel-shading and certain other uncommonly used choices. They may sell some people while turning off others, but mentioning them ensures that people know what they’re walking into.
Although it’s not as much of an issue they days, control and camera interfaces are important: is the HUD (assuming there is one) too cluttered? Are there rotes upon rotes of unnecessary sub menus when one would do? Is the camera overly finicky, making fights and puzzles that should be simple tedious and frustrating? What about the controls, assuming they’re not mapable are they intuitively designed, or will most people find themselves fumbling trying to get used to a scheme that should be simple to grasp from the outset.
Also useful, if perhaps somewhat unfair, might be comparing it to the games that currently define the genre that the reviewed game is a part of and seeing how it stacks up. You have to remember that no game is ever going to be perfect - the top games of any given genre only remain as such until something comes along to replace them that performs better and sets the new standard. It’s incredibly rare of course, but it can and must happen. This already even takes place when it comes to series that have more than one instalment, which covers a lot of games these days. There should never be regression from a high point, even if the high point was the very first game in a four or five game series.
All of these things are capable of being written about while keeping your own personal feelings about any given game or genre aside. There are of course questions of whether any given amount of time you play will be equal to an average gamers time, given that after a while of reviewing you might both become very thorough in your playing time, but also much faster than average due to the need for a high turnaround. This aside though, these are the ways to keep a review objective. Whether or not a nearly completely objective review is a good thing on the other hand…