In the dim reaches of the past, I did film reviews (even compensated) for a 'zine.
I quickly learned that, while it is certainly fun to savage a bad film, or to plunge off into some obscurity, my (hopefully) number one priority was to equip the readers of my review with what they need to decide whether or not they want to watch a movie. Just as a crude example, Joe Bob Briggs frequently reviews drive-in cinema, and at the start of a film review he will tally up breasts, decapitations, "fuzzy wangdoodles," and the odd types of martial arts you might see in a grindhouse flick (should someone be killed with a toaster, this is toaster-fu), and so these can serve as kind of metrics for drive-in movies. If your audience is a horror crowd, you might talk about practical f/x versus CGI, jump scares, black cats hurled into the screen, cheap tricks like loud scores, and "Did the movie scare me?"
Later, though, you can get into the "critical" business, as sort of a past the fold realm where spoilers rule. You might even -- and here those with fond memories of Dead Poet's Society might flinch -- what the director wanted to accomplish and just how close he or she came to doing that. You can nerd out as much as you like, but ... not until you have done the job of equipping your audience with a way to make decisions.
This naturally is the point where subjectivity rears its head.
You have two defenses against this. The first is your basic film summary, the TV guide version. One or two sentences describing the setup, drop the names of one or two relevant actors, year of production, a genre (likely split down the middle), and the dangerous assignment of stars.
The second defense is more subtle, and requires that your readers understand you as a reviewer, which is to say "wear your biases on your sleeve." As an example, read Ebert's review of Leon: The Professional, and then pause for a bit and ask yourself how Ebert might react to Chloe Moretz's character in Kiss-Ass before actually reading said review. If you knew ahead of time that he would find that concept pretty objectionable, well, then Ebert has done his job, he let you know what he cannot be objective about. Here, it is about developing that relationship of trust between the reviewer and the reader.
After this, you can talk about the progression of the careers of the director, how this screenplay probably lifted elements of something else, how a given actor is up and coming, and the suitability of the selected aspect ratios. You can get pretty deep in the weeds with that.
The job itself is as easy or as hard as you wish to make it. If your concern is only the "messaging" of the movie for whatever your political interests are, it is a breeze. Similarly, if you just want to trash a film -- and the joys of MST3King something which has wasted your time are not to be underestimated -- your effort comes largely in crafting whatever jokes you might make at the film's expense. If, however, you want your readers to feel like they can read at least the spoiler-free portion of your review and trust that you have given them enough information to decide whether or not they want to spend their time and money on that movie, well ... it is a bit harder.