So my copy of the Deluge expansion for By Fire and Sword arrived last week. Last night I finally dug in and read it from cover to cover, so I'll put my thoughts here.
The first 35ish pages are updates and clarifications to core rules from the main rulebook. First we discuss the movement phase, mainly focusing on revealing orders and working through charges, charge reactions, and counter-charges. Nothing major that I think we've been playing wrong, but it's good to have it spelled out and simplified.
The next section talks a bit about terrain, going over in depth crop fields, plowed fields, and roads...some of the more common terrain pieces that needed some more in depth explanations.
Then we have some updates to special rules. Rifled Barrels got an effective range of 20cm (still a max range of 50cm), so they will certainly become nastier to face. Elite Cavalry changed somewhat. They now go base to base like a close order unit would, but still move in the free "open order" style and have the bonus against shooting. However, they did clarify that they can only have an impetuous bonus of +1 when using the formation, and in order to get the full +2, they must charge in close order. However, there is also an example of a unit charging with the Elite Cavalry formation being able to get units from the rear into combat, which is something I wasn't aware could be done. So that's a nice boost.
Pike and Shot rules got a complete overhaul. Basically you now form "Battalions" with the pike, musket, and cannon units that you want to include. They will all be required to stay close to each other to maintain the formation, but it will allow your muskets AND cannon (crew at least) to hide behind the pike blocks more effectively that before.
There's some new rules for Robbers that will actually make the rule meaningful in Skirmish level games. There's also a new rule "Commanded Musketeers", which allows some musket units to effectively become independent skirmishers, but it appears to only come into effect during division level games.
A major rules change involves commanders ability to join units and participate in combat. They stated they wanted commanders to be less of a fighting unit and more of a command unit, so that's the results they have achieved. Commanders joining a unit for combat will now only give the +1 attack dice and +1 morale, no longer counting as an additional base in the unit (for both stats and numerical superiority). In addition, they now can die after two or more "0" armor save rolls instead of three or more.
Last but not least, there's some rules for medium artillery and their extra crews and fortifications. They should only come into play in division level games - I can't think of any skirmish list that allows medium artillery.
The next 45 pages or so cover updates to the skirmish scenarios. There are now 10 (instead of 5), plus a bonus convoy escort scenario that requires pre-planned convoy army lists. The original five are still there with a few tweaks, and five new additional scenarios.
There are two new classifications of army - "Small Skirmish Force" and "Defensive Skirmish Force". The Small Skirmish Force includes those armies that just don't get very large, such as the new Militia lists. They are only designed to play against other Small Skirmish Forces or regular skirmish lists of 7 FSP or less, as they points difference between them and a regular army could become too unbalanced. The Defensive Skirmish Forces include the armies such as Courtland, and they gain some rules to allow them to always defend and have a greater chance of picking scenarios like "Defend the village".
In the past, we've had the rules that allow small lists to gain advantages to play fairly against larger lists, and they have been tweaked a little bit, some effects being stronger and some weaker. In addition, there's now also a morale benefit to playing the larger army - depending on how much larger one army is, they can a certain amount of reroll tokens that can be used to reroll various effects. My understanding is that this came out of the Polish tournament circuit, where folks were intentionally trying to bring the smallest army possible to game the system, so now you have tactical bonuses given to extremely small armies and morale bonuses given to extremely large armies, so hopefully folks will end up more in the middle now trying to avoid giving their opponent either one.
Finally, Reconnaissance Points have changed. You still count them up and compare in the same way, but rather than the standard 4/8/12 points difference and preset effects, the army with highest reconnaissance now gets the difference in points that he can spend on effects of his choosing, and there are a LOT more effects to choose from.
Most people are already familiar with the army lists, which covers the last 2/3 of the book. It includes updated lists for Polish-Lithuanian Commonwealth and Sweden, and introduces Holy Roman Empire, Transylvania, and Brandenburg Prussia, with skirmish, regiment, and division lists for all.