I particularly enjoy the idea of a solo scenario because it allows the designer to make the game hard. Mostly scenarios have a lot of give to them because the writer doesn't know who will be walking through that dungeon door, and the groups can even be of different sizes. You can't make it too tight at that point, or some groups will just fall apart.
But when you have just one character of a known class, you can be a jerk about it! You know exactly what abilities they have so you can set up a scenario that is really tough to beat the first time through, and which will tax their resources to the limit. There is always some randomness in the game of course but knowing so clearly what the character can bring to bear is critical to keeping it tight, and making sure that all the challenges are winnable but that the character will be harshly taxed.
Going to talk about unlockable content here, FYI.
I won on my first shot with the two mini Beast Tyrant class, but it was down to my second last turn when I finally ended it. I think I have a pretty much ideal build for the scenario but I drew kind of badly so overall it seemed right. Extra special content should be rough, to let the hardcore folks have their fun!
I can easily see how the particular scenario I did would be super difficult for another build to handle. The two minis are separated into two dungeon tracks, and anyone who was used to just letting the Bear kill everything now actually has to kill stuff with the Tyrant himself. It is easy to imagine a normal build that doesn't even bother to include an attack for the Tyrant, and I think if you selected pure Bear build cards you could be effectively unable to complete the scenario.
I was mostly a Bear build so I had to really get used to actually killing stuff myself. It took a lot of adjustment for sure. I needed to get used the flow of cards and really balance whether I focused on Bear or Tyrant damage.
When I finally won, having ground through the game with a single loss card right to the end, I was extremely happy with the reward the dungeon gave me. It felt appropriate - super useful, but not broken. It makes me optimistic that the other solo scenarios will be really fun and thematic, even though they can't possibly do what I did in the Beast Tyrant scenario.
I like solo Gloomhaven. It makes me wonder if there will be more work done on this sort of thing along the road.