Hm, actually, I just thought of one possible downside. If someone doesn't get any rep they could feel like they're doing something wrong, or that no one likes them or something like that. I know that could happen anyway and
it's-just-a-pokemon-forum, but if you imagine everyone else is getting rep while you get none then that could feel bad. But yea, I think it's worth considering. I wouldn't be sad if it never happened, but it shouldn't be shot down just because a different rep system failed a few years ago.
I would assume that that drawback could be rectified if only an individual user could see his/her overall reputation. Yeah, they could say "My reputation is +100 right now, haha!", but for all you know, they could easily be lying.
Regarding a "positive rep only" system, I don't mind either way, really. Though those green bars did make the profile part of posts look purty. But I think even positive reputation would not necessarily solve the abuse problems that occur. From what I'm hearing, "Emblem abuse" is occasionally cropping up, which can (for the most part) be construed as "positive reputation". So there would still be a flaw in an actual positive-rep system.
Personally, I think it's pointless. The post comment option works fine. If people don't want to use that because
there is no visible sign of a positive reputation, then I think reputation isn't the problem here. If you don't care enough to give out a post comment without a number, then I'm very apprehensive about the reasons you would give them out with a number attached to it. The original intent is gone.
Of course, it's disappointing not enough post comments are given out, but that can be remedied without having to resort to reputation, i.e. perhaps a simple community announcement, making the post comment icon more noticeable, giving them out more yourself, etc.
You make a good point in your first paragraph...I guess users typically want results of their efforts in a
visual manner, and noticed by more than one person. Then it would seem like the time and effort it took to create a post comment was worth something.
The implementation of a "Like/Dislike" system (akin to Facebook, I guess) that is applied
per post seems like an even better idea. I'd opt for a 'like-only' system, but that would be trying to brush away the fact that there
are other/differing opinions in the world. If these likes/dislikes were to be all totaled in a single average, only the user could see the overall average of his/her 'liked posts-disliked-posts' ratio. Each post would still have its
own like/dislike count publicly viewable, though.