As long as karma is only for vanity, they don't really have to worry about people gaming the system or not -- it doesn't do anything.
In essence, it would give the Reddit staff much needed control of the content and discourse. If they one day wanted to steer the site towards only cat pictures, they can easily do that by selecting those users to be the ambassadors.
Right now it seems that Reddit is suffering from Eternal September x 1000, and they're at the mercy of the masses. But the geeky, thoughtful content that brought people to the site to begin with is nowhere to be found anymore. It's being drowned in memes and cat pictures.
I disagree. IMHO it can be accomplished with the correct structure. The problem you identify is that subreddits are shared spaces, and the primary signal that they rely on is the sum of all users. As subreddits grow, the signal becomes worse. Weighting the votes of users could help, but then you are implementing a moderator bias, and not being honest with people about their input would almost certainly lead to blowback.
One of our goals with http://hubski.com was to avoid this problem. Our approach was to avoid shared pages, and instead allow users to build a feed by following other users or tags. That way we don't have the problem of 'keeping bad content out' to begin with. Instead of voting a story up a page, users share the post with those users that follow them. Posts propagate from user to user. Therefore content cannot be 'buried' by competing with other posts in a shared space. Of course, we are not anywhere near the size of Reddit, but it has been working well so far for us. Each user is their own moderator.