I comment on over half of the stories I read (estimated, might be more but I don't THINK its less, then again I could read more than I realise, LOL), there are good reasons why I don't comment on the others:
1. I'm reading a ton of stories by that author (you know who you are, LOL!) and I'm not repeating inane comments for the ones which were just average as opposed to good or great. If I find myself descending into inanity then I have to stop making so many comments, inanity is something to be avoided at all costs
2. I detest parental spanking or anything involving children (there have been very rare exceptions to this), I also dislike F/m and M/m, purely because they do nothing for me as a submissive woman, sometimes I might start reading a story and then find out partway through that its of a type I dislike, I don't like to make detrimental comments purely on the grounds of taste too often, although I can't help myself with the kiddie/parental stuff occasionally (its why I avoid it).
4. I dislike stories where I think lack of consent is an issue (it isn't always, depends on the story) and where "punishments" are ridiculously severe and/or actually abusive, again I don't like to make too many adverse comments on stories of this type, I'm more likely to do it with an author where I like the majority of their work than someone I've never read before.
5. The story was just, meh, for want of a better word (yeah its late and I'm tired), not dreadful but by no means good, I can't see that I have anything that useful to say in this situation, I'm not an author (well, the occasional poem maybe...) and wouldn't presume to give writing advice to someone who is.
So, there you have it, as for the ratings system idea, I'm torn on that, this isn't site that appears to attract trolls but I do think it might result in less actual comments, I think it would need to be an extensive ratings system on different aspects of the story (rather than just a mark out of 5 for the whole story) to be of any practical use to the authors