Of course, theory is theory, and right now, the Zarkmark directory is pretty empty, so odds are that for any given player you look up, you won't really get much feedback. But you never know -- if players jump in and populate the site (and it wouldn't help to have a little help from them -- instead of just an Armory link, it would be nice to see a player's gear and achievements right there on the page), we might eventually get a pretty accurate picture of what someone's reputation looks like.
In fact, I'm a little surprised that Blizzard hasn't ever considered a reputation system in-game. Xbox Live carries one off pretty well -- even though I've never actually used it to consider who I do and don't play against, I have rated players and I know it's very easy to see scores if I wanted to. Given the rising numbers of VoA ninjas and PuGing in general, it might be worth it for them to give each player a socially-created rating in the LFG interface.