I think not having a negative rating is a good thing. People get obsessed with the negatives and that obsession leads to the things you mentioned. Not to mention, posting about negatives is pretty much a way of getting even more negatives. Better to just focus on the positives and let the negatives fade quickly (many people learn pretty fast if they stick in this community).
I love the idea of tags for individuals. I don't know about the aged tags. Yes, there is a point where someone's skills are not as up to date as others, but there are some skills which don't change that much. There are some new techniques, but for the most part C itself hasn't changed much. Just the libraries and tools wrapped around it. I guess it would just matter how fast those tags decay.
I like the flexibility and crowd-sourcing of this system. Yeah, it probably will get a fair amount of chatter as people assign random tags (sux0rsHard!) assuming you allow arbitrary tags. I would consider either moderating tags themselves or allowing people to ignore tags under a certain usage (only used by X different people or something like that).