The animation is great, the action sequences are really good, and I enjoy a lot of the humor and the little references, especially the blink-and-you'll-miss-it ones. Most of the time it does references really well, where if you get it it's a nice bonus but it doesn't linger on them.
The creature designs are consistently really cool.
The relationship between the main characters is cute, and I like what I've seen of the side characters so far.
More than any cartoon I've seen in the past decade, it makes me feel like a kid on Saturday morning again. Each episode is just a fun ride.
Also the entire Castlevania episode rules.