Well, I believe that the sound quality, level design, and writing alone are much higher than most indie titles on the eShop right now, but the fact that the gameplay is insanely addicting is also a nice bonus. The game supports a nice speedrun feature that I'll be dipping into a bit later, but all-in-all, it's a great game. Better than Shovel Knight, in my opinion, and I can't really say that I'm a huge Shantae fan. It shows that Wayforward took their time with this game in all the right ways in the same way that it's curious Shovel Knight took its time- Shovel Knight ultimately feels like a lot of recycled ideas with a kicking soundtrack while Shantae feels like the ultimate realization of what Wayforward should make- and really what they've always been good at. Shameless fanservice, of course, but it's self-aware without being too obvious. The soundtrack is wonderful. The gameplay is great- it feels immensely satisfying to get a new powerup because of all the things it allows you to unlock.
Anyway, I've probably said too much already, but Shantae is worth 20 bucks.