Please take a look at my full review here: https://steamcommunity.com/app/597180/discussions/0/3368153731097271418/

Verdict: highly recommended!!!

Updated review as at ~100+ hours. Full review at link above; summary and key points are listed below.

Introduction
I've noticed a frequent complaint and framing regarding "what does Old World have that Civ doesn't?" and "why should I play OW instead of Civ?". Frequently the framing is around: what does it do differently?

I do not see Old World bringing many revolutionary things to the table, or many outrageously unique innovations. Pretty much most of what it does, has been done elsewhere. But that shouldn't be a detractor!! The greatest (and most popular) things are seldom the first version or the prototype, and more often a derivative released years later, once all the elements have been tweaked considerably. For example, the iPad, Microsoft Windows, Toyota Corolla, the Star Wars franchise, etc... none of these were the first to do what they did; but they were perhaps the first to do it right, or to do it best (and let's set aside what our personal opinions may be on any particular one of these products; it is inescapable that they were indeed successful, which is the point here).

I believe Old World is like that, but pertaining to a turn-based strategy game of empire building. What it excels at is exactly taking things that have already been "invented", taking things that already exist in other games, and changing and combining them in ways to achieve the ideal balance of game mechanics and design. I'd like to go through a few to highlight how Old World "recycles" a lot of game content and why that is not a problem, but something to be valued and celebrated, especially in the way it's delivered within the game.

1 Unit per Tile
Arguably people will think that this was invented or at least popularized with Civ5. There are also realism considerations. But despite all of that, having 1 unit per tile is just more fun for tactical play. Old World has latched onto this innovation and uses it to make for fun gameplay. Furthermore, OW has leaned into this further by allowing units to cause all manner of extra/splash damage, like the spearman hitting for minor damage the unit directly behind its target. This allows the player to arrange all manner of fun tactical exploits.

Settling Cities
Obviously being a civilization game with the ability to settle new cities is nothing new. Likewise, having restrictions on where cities can be placed, and outright restrictions of exclusive city locations, is likely not new either. OW strikes the balance here of restricting city site locations, with some minor tweaking (you can settle on the site or any of its attached urban tiles).

In my opinion, this is an excellent design choice because it eliminates the extremely large degrees of freedom to making decisions regarding where to place cities, or strategizing thereon. Now you just need to choose which city site to go for, and which tile of them you wish to settle (which then highlights what the borders would be).

Furthermore, OW leans into this further by allowing city borders to expand indefinitely. This again removes non-value-added player analysis and agonizing over individual tile details by allowing just about any resource to be reachable with some work.

Dynastic Components and Relationships
As it's well-known, the game Crusader Kings is focused heavily on building your dynasty and on maintaining relationships with the various nobility around you. In fact, CK does this much better if that's the only set of game elements that interests you.

But what OW does is that it still produces these elements in an immersive way - you get to roleplay as your leader, you get to roleplay your nation better (as opposed to Civ where you're some immortal ghost ruler telling all the people what to do), and it's more historically immersive (nobility was in charge back then)... yet OW does this without making the system as incredibly complicated as CK2. Obviously from a meta-development perspective, they shouldn't, because CK2 has already done it. And from a personal gaming perspective, if this is what you seek, you can just play CK2.

What OW brings to the table is a lighter version of that so that you get most of those benefits (e.g. roleplaying and immersion), while precluding inordinate complexity.

Orders
Arguably one of the genuinely novel things OW brings to the table is the concept of "orders" being limitations to how much you can control all your units. The purpose behind this design choice is present elsewhere - preventing the player from being overwhelmed with the amount of available command decisions by creating limitations such as "supply limits" (think in StarCraft where you need more Supply Depots to build more units, and even then there's a hard max).

Like the limited city sites, this at first sounds like something that isn't that great for the player, but it turns out to be fantastic. It prevents the player from being incentivized to build such enormous amounts of units that they become bogged down with decision fatigue and so overwhelmed it starts to feel like a chore. And it does it in a way that is manageable and allows the player to improve upon as the game goes on (i.e. you can increase how many orders you get per turn through your various player choices).

Undo Functionality
With the introduction of such an orders system, the game incorporates an "undo" button, allowing you to take back any orders you issue. I see this undo feature as emergent from the orders system; with limited capacity to issue orders, it is only natural that one may fall short of one's grand designs for a turn, and may wish to undo some orders and do certain things differently (there is even an option to undo an entire turn). This does open the door to potential exploits, but it is the player's choice as to whether to use this feature or not. The feature can even be disabled permanently for a game within the game setup options.

However, this undo functionality reveals a quality-of-life feature that is so invaluable, it's difficult to return to other games that lack it. Sometimes we misclick, or remember a plan that would have needed a different sequence of orders, or have just plain changed our minds regarding what we want to accomplish in a turn. This will occur in all manner of turn-based strategy games and will be difficult to address. To have the ability to undo is extremely helpful and enjoyable, allowing players to correct those mistakes or change those actions without having to reload a previous save.

In Closing
I could probably go on and on about all the things Old World does that are nothing new, and that have been done elsewhere, and in some cases, done better. But as you peer into what makes OW so enjoyable, you feel it's almost like the game designers of OW have looked into "what is it that players enjoy when playing a game", picking those out and including them; and also looked into "what is something tedious or unfun that players are stuck having to do in such games", picking those out and finding ways to remove them from the game. For example the tedium of multi-factor dynamic city site selection (eliminated via select city sites). For another example, managing countless units (eliminated via limited orders).

What OW brings to gaming is tying in all these game mechanics and elements in a way that optimizes player enjoyment based on its game design choices. While that's ultimately a subjective assessment, for players like myself, I think it's pretty much the perfect balance of having good mechanics that are enjoyable while excluding tedious and otherwise uninteresting elements.