Generally I don't think I have the insight necessary to tell what's good game design and what's not in most cases, but I'll give it a go with pic related.
Something interesting about the game to me is the way it handles QTEs. It's not just as simple as pressing a button when it tells you to as it is in every other game I've played - you have to draw the specific pattern for the required weapon before you can do the QTE, with the player being awarded more points for drawing a larger pattern. It introduces more interactivity and a bit of depth to what would otherwise be a pretty mundane activity, forcing players to decide whether they want to get it over with as quickly as possible and draw a small shape or if they want to get the best ranking possible and draw a larger shape, the latter of which incurs the risk of taking too long and failing.
Something that's a bit of a double edged sword is that you have to buy two skills that are pretty vital throughout the entire game, Unite Guts (which functions as a parry/block) and Unite Spring (a dodge). The fact that their prices are both noticeably lower than the rest of the skills indicates that they're going to be important, but I do wonder if the fact that you don't start off with them by default lead some who maybe weren't so familiar with the genre to ignore them until later on in the game, which would make the game even more difficult.
Another thing I liked about it is that, as far as I remember, there are no mechanics that are introduced as a one-off event and never used again. Every deviation from standard gameplay is used at least one more time with increased difficulty later on. The boxing mechanics introduced in the mid-game are used in the penultimate chapter against a considerably harder boss, the second shmup part throws a lot more enemies at you albeit with fewer obstacles to dodge, a part where you have to control your ship as it dodges obstacles is reintroduced later on and has you controlling two ships at once, and so on. There's a particularly interesting instance of this sort of thing early on. One part of an early chapter has you controlling an enemy ship's movements on the TV screen by moving about with characters normally on the Wii U controller's screen; you have to do this again not long after, only this time, you have to steer the ship on the TV screen in the same manner while also having to fight enemies on the Wii U controller's screen. It's very interesting and almost like playing two different games at once.
There are other things I love about this game, too. I've always loved 'rival' bosses like Vergil in DMC3, Dante/Credo in DMC4, Jeanne in the first Bayonetta or the Masked Lumen in the second, so getting to fight Prince Vorkken four times was a real treat for me, especially since he gets new moves of his own each time. It's always a nice way to test the player's skill, and it helps that his theme is so good.
Speaking of music, the last fight with Prince Vorkken, iirc, is the only bossfight in the game in which the track 'Tables Turn,' doesn't play as you're about to win the bossfight. Really makes the fight feel all the more climactic.
I could go on, but I'll end there for now. I love this game.