Action - the swordsmanship scenes are well-drawn and the fights are pretty exciting. The reasoning behind the techniques is also explained and it kinda makes sense, at least in the context of a manga where the characters can leap many times their own height into the air.
Characters - wow, almost all the characters are well-developed and have a good back story to them. There is almost no one who is genuinely good or evil - everyone has a reason they developed the way they did. This is one of the few manga I've read where I have many favourite characters, some of whom are (at least originally) in the "opposition".
Plot - the story is really pretty well-planned (unlike in the anime where the filler arcs are yuck - and don't get me started on Seisouhen). The introductory section where Kenshin-gumi are getting together is fun, and the two major arcs are really stunning in terms of creating suspense and making little twists to the story.
Humour - Although it's mostly a drama, there are deft touches of light humour, especially funny drawings of Kenshin going "oro?" - never fails to make me laugh.
Historicity - I tend to like historical manga because it gives me an incentive to learn more about Japanese history. No different with Rurouni Kenshin - I know a lot more about Meiji-era Japan and the Bakumatsu than I did before (which was zilch!). There are also little explanations of the actual historical events and characters, so there's a nice mix of history and fiction there.
Re-summary - read it, you won't regret it. Seriously good.
