Let's start with mechanics. As I mentioned in the review of Fear Itself, default GUMSHOE fits pretty cleanly into the "rules light" paradigm, as seen by the size of the book and the percentage of that book dedicated to game mechanics. Moreover, especially in the early generation of GUMSHOE games (Fear, Esoterrorists, Trail of Cthulhu), the focus was on getting past the (often hyper-deadly) action as quickly as possible in order to get back to the business of solving mysteries. So, on the surface, GUMSHOE seems like a strange fit for a swords-and-sorcery game that is presumably going to have an action focus. But, if you stack lots of mechanics onto top of the GUMSHOE engine, you lose much of the benefit of the clean, fast GUMSHOE engine.
This problem has been addressed before in Ken Hite's Night's Black Agents, and you can see the influence of that game clearly on Swords of the Serpentine. The biggest add is that each of the General Abilities now has a "Booster" (a "Cherry" in NBA-speak) if the PC has a rank of 8 or more in that Ability. So, Warfare of 8 allows you to attack multiple targets; Bind Wounds of 8 or more removes the normal penalty for healing yourself, etc. This sounds really simple, and it is, but it is a very elegant structure that makes characters with different arrays of General Skills feel very different. It works so well that one of the bits of feedback I got was the playtest characters that spread around their General Abilities and don't have Boosters feel a little dull in comparison to the one's with Boosters.
The other big move that pumps up the action level is that Swords of the Serpentine works on the explicit assumption that players will use their Investigative Ability spends to enhance their General Abilities, and provides very clear guidelines for how to adjudicate those spends. Previous GUMSHOE games tend to keep the world of Investigative Abilities very separate from General Abilities, and I would be hesitant to let players in most GUMSHOE games go hog-wild with using spends to get a combat advantage. But it works and makes sense here, and it makes PCs feel heroic without really changing the way the system works. It's subtle, but has a big effect on the way the game feels.
One departure from Night's Black Agents is that Swords of the Serpentine dramatically cuts down the number of Abilities, both Investigative and General, on the character sheet. In addition to simplifying the on-boarding process for players, this move also enhances the genre "feel" of the game. In something like NBA, drawing distinctions between different kinds of spycraft-oriented knowledge or ways of killing foes feels in keeping with the feeling of being a super-spy with multiple hyper-specialized areas of training. Whereas in Swords of the Serpentine, the "Warfare" General Ability covers every conceivable way of making physical attacks against opponents, which is in keeping with the idea that Conan can do his Conan thing with whatever happens to be available to him.
The Abilities that do exist are dripping with atmosphere. The "medicine" Investigative Ability in Swords of the Serpentine is called "Leechcraft," which is so great. The "drives" mechanic in Trail of Cthulhu and other GUMSHOE games is re-interpreted as defining three answers to the question "what is best in life?" That questions comes from the legends of Genghis Khan, but it is best known from a scene in Conan the Barbarian:
I love every bit of this--(1) it's very genre appropriate; (2) it's very funny; (3) it's a really good way for a player to get a handle on what their PC is all about.
But the atmosphere is not just generalized fantasy, or even swords and sorcery fantasy. Swords of the Serpentine is situated squarely in a setting, and that setting is every bit as compelling as the rules material. Right off the bat, the draft document gives you what I think is the ideal amount of setting detail, enough to communicate a clear sense of what this setting is about, without constraining GM freedom of action and creativity. In this way, it is very similar to the default "Dragon Empire" setting in 13th Age--it draws the map for you, but it leaves plenty of blanks.
What is defined for you is primarily the city of Eversink. As the name implies, Eversink is built on a swamp and is slowly sinking. Venice, in all of its medieval glory, is a clear inspiration here, with canals functioning as streets and gondolas being a core means of transportation. Eversink is portrayed as a classic fantasy "mega-city," in the vein of Lankhmar and its inspirations like Greyhawk, Waterdeep, etc., so it is big and crowded, with many factions, conspiracies, and intrigues.
Speaking of factions, they are built into the rules and PC design. Each PC begins with relationships to three of the major factions of Eversink (two as Allies, one as an Enemy), and these relationships function like Investigative Abilities, allowing for spends out of these pools (as well as pool from which the GM can make spends against the PCs, in the case of Enemy factions). This system has some similarities to the Icon mechanics of 13th Age, but on a much more grounded level, in keeping with Swords of the Serpentine's lower power level generally--rather than the undead Lich King of 13th Age, Swords of the Serpentine gives you the Guild of Architects and Canal-Watchers, a secretive clan that is pretty important in a city that is sinking into the water. The faction mechanics allow GMs and groups to play a gritty, politically-oriented game. If you asked me which ttrpg system I would use to run Game of Thrones or a Game of Thrones-like story, I would go with Swords of the Serpentine. But the faction mechanics are also not absolutely essential to play, so if the group wanted to go the other way and just have high-adventure without any political elements, that would also work without having to rebuild the system from scratch (unlike, for example, the Icon mechanics in 13th Age, which have their roots dug pretty deeply into the game system).
Another defining element of Swords of the Serpentine, one that merges rules and setting, is the sorcery system. Sorcery, by definition, involves consulting with dark powers--a sorcerer makes a pact either with a small god or demon, or otherwise gains forbidden knowledge from the lost Serpentine civilization of the past. In order to learn Sorcery, your PC must take on Corruption. Corruption can be spent to boost your spells, at the cost of either taking on physical deformities, or spreading that Corruption out into the world. As far as a mechanics of Sorcery, the Sorcerer chooses one or more Sorcerous spheres to represent the manner in which their Sorcery manifests. So, if a Sorcerer has the Luck sphere, he or she can produce magical effects that manifest as extraordinary lucky events; if he or she has the Stone sphere, his or her powers can manifest as creating or affecting rocks or earth. This system gives players enormous room for creativity (if you squint hard, it has some resemblance to the magic rules in the White Wolf Mage games), while still imposing meaningful restrictions on the power of the PC and without requiring an massive rules apparatus for Sorcery--the entire Sorcery rules are 30 pages of single column, un-layed out text in the playtest document.
Finally, I want to talk about Denari, the patron goddess of Eversink. Actually, patron goddess sells her short, as Eversink is literally Denari's body made manifest. Denari is the goddess of commerce and civilization, and every mercantile transaction is a prayer to the goddess. The brief section describing Denari and her church makes clear that she is the embodiment of capitalism as a religion--the priests and priestess of Denari preach on how to best calculate gross profit margin, giving away something for free is a sin, etc. Here, I think, we see the strong hand of Emily Dresner, writer of the truly outstanding (and hilarious) Dungeonomics blog about the economic challenges of being a fantasy hero. This dimension opens up space for Swords of the Serpentine to work as satire and social commentary. As someone who believes that modern capitalism is, in fact, a religious belief system, this is like catnip for me--pump this directly into my veins.
I know I'm gushing about this game, but it is genuine--I cannot wait for this to come out and get this in the hands of more people. It has all the simplicity of GUMSHOE, but with interesting and fun mechanical elements and action potential, wedded to an interesting and compelling setting that provides for real depth and complex story-telling opportunities. Every part of this feels like a massive hit, and I have no problem acting as a shill and evangelist. Be on the look out for this game in the New Year.