For me, it is definitely tagging. As much as I like Kudos and Accepted Solutions, I'm more of an "organizer" so tagging is my favorite feature.
I think "Search" still has a ways to go on so many applications across the internet. Google does it really well, but most other apps pale in comparison. Tagging helps fill in where Search falls down, in my opinion.
Also, I like leaderboards too. Tagging leaderboards are cool because assuming somebody is tagging content with approriate tags (not non-sequiters), they are improving the content of the forum without actually having to post. So if there are avid readers of your community that do not post much, if you get them into tagging, they can really improve the place without actually having to post.
I really like the accepted solutions function and found that our community quickly picked up on it. We are getting ready to deploy kudos and I see how they can be customized to a board level, at least in terms of the graphic which could provide for some diverse use scenarios of the basic function.
I'm coming to appreciate the complexities and nuances of the rank and roll structure, and while we aren't yet taking full advantage of the formulaic capabilities, I can see how these coupled with some multi-threaded rank progressions aligned with key user behavioral archetypes could really help cultivate user behaviors. And of course, reporting is very good and getting better all the time.
There really is a lot of depth to the platform, and I think the subtle capabilities become more compelling over time. It is taking me a little while to get past the high level features. It's like watching a big action movie - the first time you see it, the stunts and action define the experience, but the second or third time through you begin to appreciate the character development and some of the secondary plot elements, and the story gains more depth.