Grain of salt... pound of salt... who's counting? 
I think there are 4 factors:
(1) There is no standard for a review. Some people post what amounts to a personal diary or travellog. No problem with that, but it is less useful for assessing the ship and making plans. (In fact, I find these more interesting than a dry, emotionless review.)
(2) As mentioned above, some people gush or complain about the bad trip they were probably determined to have from the start. Aside from we @ddicts who like to routinely write reviews, these two groups are probably greatly over represented in respect to the average cruise experience.
(3) Some people allow one event to ruin their cruise. Now, if you're one of those people who are prone to that type of outlook the review might be useful, but for most of us not so much...
(4) Most of us are just poor writers. I know many people with higher education, some even with a master's degree, who just can't communicate well using the written word. The intention may be for a very useful and balanced review, but that is not the result.
FWIW, I think reviewing based on what the line advertises versus what they provide has very limited value. If they neglect to provide something basic, for instance closing the buffet and providing no other means to get food easily through the day, that is fair. If you critique based on the oversell that every company in America does (e.g. "A million ways to have fun on every Carnival ship"), then you're telling people something they already know.
