I'm not sure where you get that impression from, as far as I know your rep doesn't affect your ability to post at all.
I haven't been on this forum for few months because of it. :/
If your rep is bad enough I guess that would cause the mods to take a look at your account as a candidate for banning since you should only get that low by obvious trolling and bad forum behaviour.
Just to be the devil's advocate, since I like arguing(probably reason for negative rep in the past), or making bland points that really are for ideal conditions. Point: A whole group of people can just decide to pick on a person for no reason, and handout negative rep to that person.
You also can't take every neg rep as an affront to your personality, this is the internet, people do mean things on the internet. That shouldn't stop you from expressing your opinion as long as it's done in a respectful manner.
As you said, it shouldn't. But I am sure there are cases where negative rep has been handed out unfairly/system's use misunderstood/system used wrongly/personal grudges, and I'm not talking about any of the stuff/arguments currently in this thread. Some people are more prone to heed others opinions of them(this can't be denied, as its heard of all over the media often enough) and simply might not voice their argument/opinion or stop being active.
Onto a road closer to the topic,
I'll look into some sort of achievements, I think there may be a standard feature in vB that can facilitate this. As I've mentioned before, we do not use 3rd-party vB plugins because those tend to break everything when there's an update - not worth the risk.
As for the karma system; I'm starting to wonder if it's worth it as a form of interacting with one another. The core purpose of the forum is for discussions through posting. If the karma system is somehow negatively influencing the way people post, then maybe it should just be turned off for the time being.
I've seen reputation systems like this cause big problems on other forums; sometimes keeping things simple is the best course of action.
It's bad enough that post count becomes sort of pecking order measuring tool, and now it seems karma has been thrown in to the mix.
I would have loved to give a link to a forum that made use of an automatic rep system, by setting and checking milestones. Unfortunately, there was some sort of vbulletin plugin for linking to a social site that had a security flaw, even the patch didn't fix it, so they got hacked twice and now using mybb... So no more example..
But here's some details I remember: It had Awards, and Achievements tabs, was point based, and points were given for posts/threads and some other stuffs. It had a thanks button under each post, so you could thank and this gave some points. The awards were 'mysterious', such that it didn't say what you needed to do to get the award, and the award could only belong to one person at a time.
Some other things: Referrals got you an award/achievement after a certain number reached. Number of friends. Number of thanks. Time registered, up to 1 year(thereafter its pretty pointless). If I remember any more, I will put it here. Can't say if it was 3rd party or a built in vb function. Does this site use vb? If so, why isn't the copyright at the bottom? But yes, now you mention it, it is vb, since it looks and works like vb.
