What makes it stand out from other samurai, ninja, or action series is the fact that RK is rooted on a meaningful plot. There's always a good reason why any form of violence takes place. The action is extremely detailed. The different sword and martial arts techniques are given a lot of emphasis and are explained very well. RK creator Nobuhiro Watsuki really knows his stuff and he dazzles the audience with this knowledge.
Characterization is superb.
Kenshin Himura is one of the best characters I've encountered. Kenshin's sword skills are mesmerizing to behold. The supporting characters and villains are just as well-developed. Virtually everyone has their own in-depth history, which in turn explains their motives and actions. The series is propelled more by the characters rather than the plot.