The lack of standards, and the role personal taste plays. By rating a beer I am logging (mostly for personal usage) how much I enjoyed a beer. Therefore I do not have to know about the style, be unbiased by personal views and preferences, or be "qualified" to rate and I find it slightly ridiculous to suggest as such.
There may be no flaws to describe, but personal taste. This isn't a contest where each beer is judged against a standard, this is one person's personal impression of that beer.
Of course personal taste is a factor, however it is much more useful on styles you know.
For instance I would be able to taste how a certain malt or certain hop comes through in an IPA and would be able to give useful criticism on certain aspects. I think it's good to balance personal taste and objective observations. While there is no objective standard to be judging beers on, I don't think it's bad to consider how much a beer aligns or deviates with "traditional" tendencies of its' style. Often times the beers that deviate from the tradition of their style can be my favorites!
However I do think that if someone is going to give a bad rating they should at least attempt to explain why. I don't think concrete knowledge is as important as gathering experience and familiarity with various styles. There shouldn't be an exclusive right to rate beers, and I hope I'm not appearing to suggest that. I just think these brewers deserve a bit of our effort if we choose to rate.
The thing is, many of these assume that people rate and post tasting notes for others to see. That is in many instances not the case. They don't do it because they want to improve the beer or give a genuine feedback, they're just stating whether they enjoyed it or not.
If you want feedback, you shouldn't be looking at Untappd. Maybe Ratebeer is a better option as its users are on average more beerwise, but in the end it's not a safe bet.
The problem here isn't people rating beers, it's trying to make sense of the randomness people attribute to beer(s).
I feel like this is also applies to real life encounters as well.
I enjoy dark beer. The more stout the better however I don't like any IPA's I have ever tried, I just don't like "hoppy" beer. So anytime I ask friend of family what's a beer I should try, no matter who I ask it's always an IPA, or a terrible domestic beer.
I think it all stems from people not accepting the different flavor of beer, I will still try an IPA just to try it. Who knows maybe someday I'll like one, but I don't stick to a type and I feel like that's what most people do. They find a type or kind of beer they like then expect all other beer to taste that way and when it isn't they thinks it's awful, instead of actually tasting and considering what the brew master intended it to taste like.
That's exactly why I like you guys, it's not all this sucks, that sucks, it's genuine interest in the intention behind the beer.