The Good
So easy to run. The fact that treasure is intentionally randomly generated lifts a burden from the GM, and adds excitement to the table when we all lean forward to see what weird treasure we find. There are also a variety of random tables to help with improvisation cues. This is a great gateway rpg system.
Foundry has everything you need, including monsters and magic items. The game really is drag and drop and handles all the mechanics.
Carousing was great. In between adventures, players can spend their gold and roll on a table and earn XP but also discover a rival or ally. This was fun to do! And a wonderful money sink. It meant that players often started the adventure with only pocket change, which kept them hungry for adventure.
Magic works great. Rolling for spellcasting feels exciting. Every time a player cast a spell, there was an audible sigh of relief when it worked. If you fail that roll, then you lose access to the spell until you rest, which is a significant set back - especially at low level. It also means that learning a new spell is a significant gain for a player - they now have another useful tool in their toolbox. Something to fall back on, if their favourite spell is lost for the day. I was able to be generous with scrolls, and was able to use some of the weirder, more unique spells (such as "Speak with Insects") that our adventures included, because players were going to be able to use them, without having to waste a spell slot on it.
Real risk of death. When a character reaches 0 hit points, they roll 1d4 to see how many rounds they live. This happened frequently, at least every session (our thief only had 3 hit points at level 2), and I added a visible clock on to the screen for everyone to see. It really felt like it added urgency to the scene, without being overly punitive.
Good selection of monsters. Most of the classics are here, and a broad selection that I could use for reskinning where needed.
Back to basics XP. For the last few years, I have been using milestone level advancement. This time, I deliberately used a trickle feed of XP. As the players finished a scene or room, I gave them XP for it. Players commented that this was a good way to run Shadowdark, as it kept the just-one-more-room urge to push their luck strong.