And in my experience reviews should only be used as a guide and not to be taken as gospel. Just because 70% of people think a restaurant, or hotel, is good does not necessarily make it good.
The devil in the detail works both ways. A positively reviewed place could result in a negative experience, and vice versa.
Also, we are creatures of limited precedent. If you have a good experience you'll probably come back even if the "average" review is bad. If you have a bad experience you may never, ever come back again, regardless of the existing precedent (i.e. you'll never come back even though in reality your experience may actually be just a "one off", though it can also work the other way - your bad experience may be so because the place is genuinely bad).
As you say, of course it's supposed to be just a guide but that's why I also qualified my response as saying that "it isn't everything". Reading some of the reviews gives you an idea
what people complain about. It's very difficult to judge people on how they take reviews and it's even difficult to recommend to others how to take reviews because of just this - it is purely a guide. But the paradox is that if it's just a guide and not that dependable (especially when those who have an opposite experience report back as such), then what is the purpose of allowing people to write reviews in the first place, because it's just making very bad precedent. (Of course, this also would be a case to put every other "professional" reviewer - e.g. film critic, restaurant critic, hotel critic, etc. - out of a job and out of business).
We also have to be aware of shills, anti-shills and businesses deleting or denying reviews. TripAdvisor - probably unfairly targeted because it is the biggest such site - has all of these.