Just finished looking through the Secrets of the Unlit Star and really impressed that it addresses a lot of the issues I had running the original beginner box for new players.
While it deviates from the classic default party composition, it is a big win for teachability sake to only include spontaneous casters in the pregens. They have shorter list of spells known so an abbreviated version of their spells can be printed onto the sheet. Additionally no need to try to teach brand new players about vancian casting mechanics. Not sure if its as drastic as spontaneous/prepared but also think it may be easier to teach barbarian and monk since their kits are more self contained (using an action to rage/stance) rather than needing one of the many strategies to try to impose off guard to get sneak attack online as a rogue.
Additionally the included adventure address a lot of complaints players have had about the original BB: an underwhelming mystery/story, a dungeon that feels in-cohesive/inexplicable, mostly linear dungeon design, and lack of social encounters. It also addresses some of my GM issues with the original like players having no motivation to keep exploring the dungeon, trying to exit the dungeon frequently/too early, and complex hazards that are a death trap.
And while overall I think its an improvement, there are some regressions from the original BB. For instance >!the adventure starts with the "you're traveling down the road to your next adventure when you feel a compelling urge to get off the road and enter this random structure" trope!< which I know would not go over well with players I generally run things for.
Overall I'm quite happy with this update and very happy that it addresses some common critiques of the BB from the community.