One thing I really like about Tildes is the exemplary tags for comments. I love being able to let someone know I thought they had a great post, and I especially like that it's anonymous (though I realize some people like signing theirs, which I'm fine with too).
One thing I've found myself wanting to be able to do is give someone an exemplary label not for any one individual comment but for their contributions to the community at large. Maybe they're consistently thoughtful and insightful; maybe they go out of their way to post a lot of content for the community; maybe they're contributing code to the platform. It's less that any one particular thing they've done is amazing (though they often have individually great contributions too) and more that they've demonstrated a noteworthy and consistent pattern of good behavior.
As such, I think having something similar to the exemplary tag but applicable to a particular user could be very beneficial. I realize privately PMing a given user can currently accomplish this, but those are not anonymous, and I really like the idea of supporting others without revealing who I am, since I don't want my praise of others to influence their opinion of me. Furthermore, for the community at large, I think there's a benefit to praise of that type coming from "a voice in the crowd" rather than specific identifiable users, as it promotes community goodwill rather than person-to-person cheer.
Of course, with any type of anonymous feedback the thing to consider will be the potential for misuse. Someone could easily target/harass someone using an exemplary user feature by writing a nasty message, but this is also currently possible with exemplary tags and I don't know if it's been a problem? Nevertheless, it's something to consider. Perhaps a built-in report feature should something cross a line?
Furthermore, if such an appreciation mechanism were to be implemented, I would strongly advocate against any sort of publicly visual indicator on the site (like the blue stripe on comments). I think applying differences to that at the user level can create an appearance of user hierarchy, which is undesirable for a variety of reasons. Instead, I feel like it should be invisible to everyone except the recipient--basically an anonymous PM that they can't respond to, letting them know that they're awesome and why. I also think a similar "cooldown" system would benefit it. In fact, I'd probably advocate that it be longer than the one for comment tags.