I ran Shadowdark on Friday and can now safely make a few comments on its from an actual play experience. Here are my observations:
The rules are minimalist in design
I was surprised time and again at how brief and mechanically simple the rules were. There's a lot of buried "implied mechanical intent" hidden away in the rules, and while Shadowdark has a veneer of D&D and OSR all over it, this means despite looking and feeling like you are playing an old school version of D&D, you definitely can't make any assumptions about this edition of the rules from older editions (or even 5E); if you do, you will get tripped up. You can house rule.....but I suggest playing it straight for a few sessions before making any assumptions. Even the one house rule I brought in to play I am unsure was a wise idea.
There are some interesting approaches that are counter-intuitive to 5E and OSR design
That last observation leads to this one. Here are five examples of design in Shadowdark that is counter-intuitive to what your brain might tell you the rules should do:
--You don't start with max hit points (you roll), and you only add CON (maybe?) the first time, not every level
--Classes don't have auto-progression on to hit at all; it's gonna happen (sometimes) with talent rolls
--You don't add stats to damage (STR and DEX), just to hit rolls; damage might be added to your roll, but only as a class feature
--Monsters have no hit dice; I didn't notice this until I was in the thick of it
Leveling up is not what you might think
When you level up, your XP counter resets to zero. So if you are level 1, gain 12 XP for the session, and level up then you are now level 2 with 0 XP, and you now need 20 XP to get to level 2; this is not immediately clear from reading the book, because your learned experience from prior editions is telling you otherwise; and the incredibly brief rules-minimalist instructions do indeed tell you how it works in the book, but it took everyone at the table a minute or two to figure this out (but not the young players who did not have older editions burned into their memories)
XP through loot and prestige awards is deeply engrained and informs play
The XP mechanic is a deliberate throwback to AD&D, where treasure begets XP, but it does away with XP for combat and encounter engagement (traps, puzzles, etc.) more or less entirely. I am sure not everyone plays it this way, but I am for the sake of seeing how it leads to the flow of advancement. It means that the GM needs to keep this in mind; a stingy GM is not only withholding loot, he's withholding advancement. Luckily, advancement includes things like ephemera such as titles, deeds, land grants and promotions; anything in which the station or social ability of the characters might improve is considered an award in this system; in thinking about it, a group could fight an ogre and kill it, discover no loot, but then tell the tale and have a bard carry on their deeds on song thus spreading their reputation far and wide as monster slayers....and the song would be what gets them XP, which is an interesting approach.
The "Cast a spell until you fail" mechanic is a fascinating limiter
So in Shadowdark you have spell slots, and you can keep casting spells as long as you keep making your caster checks. Once you fail, you lose access to that spell (and fail to cast it) until your next morning's preparation. It's a clever bit of book-keeping reduction and also allows casters to be useful until their luck runs out. I kind of wish I had conceived of this method back in the AD&D days, when I disliked Vancian magic so much I wrote an entire spell point mechanic to allow for more fluid spell casters in play.
The game really wants you to go into dungeons with torches
I actually tripped myself up at the start of the game, focusing on perhaps more plot than Shadowdark demands or needs. Next session will lead quickly to a dungeon teeming with issues (and treasure), but yeah, to contrast with my Saturday Pathfinder game (yes, I ran two games this weekend) where everyone spent half the session in a complex mystery following leads, interviewing suspects and eventually getting into a brawl when the right (wrong?) suspects were caught up to, that game would have in Shadowdark terms been considered a bit of a wash, especially given how Shadowdark's entire focus appears to be on "things happen, preferably in dark and dangerous places." There's a reason it has the torch timer mechanic....although I did feel like its loose effort to identify what ancestries (or monsters) could see in the dark was a step too far in the simplification of things for me. Some third party supplements have added drow in as playable ancestries, for example, but then fail to address whether these drow would in any universe not have dark vision? The Shadowdark book suggests some things are better suited to moving about in total darkness, but seems to hand waive it off to the GM to determine what those things are. It's perhaps a slightly better approach this way than, to contrast, 13th Age's assumption that light doesn't matter at all unless you want it to, but still - I find it interesting that some of these "we do D&D differently" RPG approaches seem to double-down on light levels being a bone of contention that must be simplified or stripped out in some manner. Shadowdark's approach of saying "light totally matters, and nothing can see without it" is a fun approach to simplification, though.
A lot of Shadowdark is spiritually reminding me of how Tunnels & Trolls plays; mechanically its closer to D&D, but the net result of its design choices (talents in classes, for example) and focus on dungeon delves (minus the treasure = XP mechanic, which is deeply antithetical to T&T's approach) really remind me of my old days with T&T.
Okay, more observations soon as we continue with Shadowdark!