There will be many lessons worth learning from this war and I would like to throw my hat in the ring with my own thoughts on what we should learn. And I would like to start with what is the most important lesson of this war. Namely that of the importance of mass.
What I mean by mass is the size of the forces you can put into the field. The men carrying the bayonet to the enemy, manning the guns, and those supporting them such as truck drivers and doctors. That alone has had a far greater impact on this war than any one single weapon system has. And so, I feel it is a disservice to try and talk about other lessons without addressing first this most important one.
Mass is unequivocally the reason that Ukraine is even in this fight at all, despite a qualitative disadvantage in equipment (less advanced tanks, artillery, air defence, radar, and equipment). Because when the Russians crossed the border on the 24th of February 2022, they did so with a force of 70k-80k strong, but facing against them was a Ukrainian army of 200k and a paramilitary of 100k. That is a greater than 3-to-1 force parity in favour of the Ukrainians (If we add in the Donbass militias to the Russians it drops a bit below 3-to-1). The general rule of thumb in military circles is that you want to outnumber your opponent by at least 3-to-1 if you are going to go on the offensive, since the defender has a lot of advantages.
The reason why mass matters are many. One of the main ones is that it allows a force to simply occupy more land. Through this occupation of land, you deny the enemy the ability to freely move around by forcing them to have to fight you if he wants to go anywhere where you are present. Something many Western planners seem to ignore when they came up with their various doctrines after the Cold War of how they could with a smaller and more mobile force merely “outflank and destroy” the enemy. But when your enemy has a contiguous defensive line, you will be forced to fight at some point. And if you win and breakthrough, you now have a supply line that needs to be defended from the enemies on either side of your breakthrough point. And so, as you push in deeper you only stretch your limited forces out more by extending the supply line you need to defend.
We saw this exact thing happening in the North at Kiev, where Russian forces had punched in deep on both sides of the Dnieper River, pushing through multiple defensive lines that the Ukrainians had. But it meant long and vulnerable supply lines for the Russians, which were hard to secure as light Ukrainian forces could just walk through the empty countryside to get to them. Or in some cases lay low while heavy Russian forces passed by them, only to come out to attack the much lighter supply forces coming later.
This is part of the reason the Russians pulled out of the North, since they lacked the forces to occupy the countryside and defend their position. Instead rotating those forces down South to help set up a contiguous defensive line of their own to face off against the Ukrainian line. A simple natural evolution of every war that goes on long enough (Another blind spot in Western military planners in the past decades). But as we see by Ukraine’s successful offensive in Kharkov of September 2022, Russia lacked the forces to properly defend its defensive line, because Ukraine was able through its much greater manpower able to keep Russians occupied everywhere, while they built up a force to strike at a weak point. And Russia lacked the reserves in the area to respond rapidly to counter this offensive. This is why Russia started its own mobilization drive, so as to reach parity in force size with the Ukrainians and perhaps overtake them.

The other reason why mass is important is that it can cover for materiel deficiencies. This comes at the cost of trading equipment and/or manpower to cover up those deficiencies. For example, driving tanks closer to the enemy to more effectively engage them while having an inferior fire control system, this will result in likely losses of tanks, in exchange for achieving the mission objective. Or to give an example from the recent conflict, where Russia is using its superior numbers in artillery systems and ammunition supplies to cover for their manpower deficiency. But being inferior to the enemy at certain capabilities is a fact of war that will always be the case against a peer opponent.
And of course, on a rather more grim note, having a large force with a lot of equipment allows you to sustain losses and continue to fight. Casualties are a fact of war, and a peer conflict is going to see heavy casualty figures (As we are seeing in Ukraine today). Many Western armies in 2022 were built with almost no materiel and manpower reserve to them, having cut those in the Post Cold-War “End of History” period as a means of saving as much money as possible, and likely would have fared no better, if not worse, than Ukraine has.
But having a large army and a large reserve aren’t enough to have proper mass, for that you need to also have the ability to supply and sustain those forces for a long period of time, not just weeks or months, but multiple years. Because while you get a unanimous say when to start a war. You do not get such power on when to end it. And to be able to sustain you need a military industry. One that would look wasteful in size to an accountant during peacetime.
The reason for this is that you can find a man to wear a uniform in a few days, train him to hold a rifle and shoot in a few weeks, and be a soldier in a couple of months. You can also build a gun, or tank, or plane, in a few months’ time in a factory. But to build a factory, the machine tools for said factory, that all takes years. It is so much easier to have a factory that can make 500 tanks a year only make 5 a year for many years and then suddenly ramp up to 500 when war breaks out, then it is to try and expand a factory that makes 5 tanks a year to 500.
We are seeing this play out right now, where Russia kept many of its large Soviet munitions factories running at a loss, largely as a job creation scheme. But now with the war they were able to quickly ramp up production to a tenfold increase in short order, with further increases to come still. While in the West we are now spending billions to expand shell production factories to try and keep up with demand. And even then, with projected production increases it will still not be enough to outstrip usage of reserves.

In short, to fight a war, a proper war. One needs a large army in the field to fight, with a large pool of reserves to draw upon. And a strong industrial base that can quickly respond to an increase in demand.
All the commentators I follow say the army the West spent eight years building in Ukraine to crush the Donbas was essentially destroyed by Summer '22.
I think the primary mistake by Russian leadership was to underestimate how ideologically commited NATO is to destroying Russia.
Nicely done, particularly the rare insightfulness of how few forces Russia initially used compared to the b.s. propaganda most western sources claimed (200-250k troops, etc etc)