People love to complain about one unit being too useless, or about someone who picks the wrong toolkit.
I want to do the reverse and mention all the fun interactions I remember.

Ships on non-ship maps

Despite all the hate ships get, they are very fun on unintended maps. So long as there is some water, making ships is an unorthodox strategy that gives no metal, but provides incredible map control on a specific land. Figuring out if we can deal against them, or if we can plain ignore them is a tough choice on some maps!

Spec ops

Taking out an antinuke, sniping singularity reactors, or plain disrupting factory production... There are so many reasons to sneak up on the enemy! Keeping units away from enemy forces, sometimes making ambushes with slow reavers, or breaching with scorpions puts a whole new element to the battlefield that is often left out in other RTS games.

Tanks

Tanks feel like tanks in this game: Heavy, tough, meant to outlast the damage and dish a lasting impact. Whenever a minotaur is seen on the battlefield, everyone switches up tactics to deal with it, or play along.

Striders

Dantes spit fire everywhere, funnelwebs are effective battle construction workers, every unit feels like they have a purpose. Even if it is to build a bunch of athenas to dump lategame metal, there is a reason to build a strider hub.

Superweapons

In lategame, if the game becomes stale, superweapons are a last-ditch effort or a new objective of the game. Silos, cerberuses, and big berthas, albeit not super, are a special objective in their own right, every time they are produced!

Shieldbots

This used to be my long gone friend's favorite factory. He used to play it so much that I'm no longer interested in playing it. Whenever someone fields it, I sometimes wish I could play along him, again.

Oh, and they feel like a bossfight to deal with, if every other unit is a creep!

Rovers

Fast and furious. Although scorchers work best when they are in low quantities, counter-intuitively, they are the scariest raider Because they are scary in low numbers! This often lets the rover player build up economy fast, forcing the opponent to spend way more on defense than it takes for the rover player to build up offense. It is a great starter factory for this reason, and there are many good follow-ups for it.

Jumpbots

The jank factory. All units from it feel unique, from anti-air moderators to skirmish pyros, to self-replicating puppies and the buffed dirtbacks: the jacks.

Cloakbots

I avoided this factory before, as I don't like basic things. Later, I learned to use this factory in unexpected ways, like a forward manufacture base! Its units are the cheapest to produce; and with plates, it becomes easy to field out an army in sheer seconds, and for cheap too.

Amphbots

People meme about this factory a lot, but it's a good factory. I used to wonder, "How is an amphibious factory this strong?" Now, I'm happy that it provides variety to the game.

Air units

Air units are one of the most unique units in the game. When most games would make them a unit with the "hover" property, air units are fast and mobile with an armor/damage tradeoff. They punish any player who is not prepared for them, and they are one of the best ways to get good map vision. They are worth getting just for transport & vision alone.

Hovercraft

I always wondered how a glass cannon army would work. This factory is it! If the enemy is not dead/severely wounded by the first volley, run.

Developers

The game is developed by the community, for community, out of community. StarCraft, believe it or not, used to be made this way. I still have some of their older notes, and they are all fascinating. Zero-K is made in a similar fashion, and it works. More-so, I appreciate every- Ah, screw it, I love every developer who worked on this game! Shaman, GoogleFrog, Anarchist? And all the others I did not mention: From mods to basegame, to things as small as figuring out all the little bugs and patches. You kept the game polished. It is clear that your work is made out of love, and love spreads love.

Players

The players, both good and bad, both legends and segends, all shape this game. Yes, even the players that blame their team have taught us all something, when everyone resigned too early. And the times the players cheered up their own team in dark peril are also remembered. But perhaps, the most respected members of the game are those who were even willing to help out their opponents, be it to figure out a better strategy, to ask about their day, or mess around instead of fighting.

The "War" aspect

Zero-K is a great game because it portrays war the way it is: Meaningless, cruel. Zero-K, in spite of all the fluffy words I said, is a war game. It should never be thought of something that is not a war game. Although fighting teaches us to cooperate, the game remains, again, a war game. However, seeing the destruction of the nukes, the indiscriminate destruction of snitches, the desolated wastelands made by scorchers, and the cutthroat environment where "luck" decides the victor have done well to make me wish that wars end again - or never come to exist.

But there is hope.

Zero-K has taught me that even in an environment of hate, there is love, and detest of hate. Teammates supporting other teammates, shame on those who shame. Even without teams, sometimes, the best strategy IS to not fight at all, or that all those who fight by the sword, indeed, die by the sword. That there is always a survivor, and that there are ways other than swords to fight: Even in a Free-For-All environment, nobody likes a fighter. That there will always be a sun tzu who will write about wars everything, including...

"The greatest victory is that which requires no battle,"
"If you know the enemy and you know yourself, you need not fear the result of a hundred battles."

Yes, Zero-K is a war game. There are war games, great war games, and there are good war games. War games play war for the sake of war, great war games present wars as great, and good war games show war in all its colors.

I love Zero-K because it did not kid-nap me about war's destruction. Even if it is just robots.

Conclusion

Zero-K is art.