they splash lots of color over a pretty decent silhouette / lineart and simple but cute and attractive character/costume designs, and the the unified saturation and value keeps it all looking smooth. the broken lines and missing aa are, as mentioned, a result of a general neglect of technique in favor of efficiency. another example of this - static heads. not really great pixel-art wise, but very effective for a game. Basically, these sprites are what we refer to in tabletop games as "regimental quality" - good looking at a glance, but hardly works of art. The same goes for all of the artwork and even the writing in game.