Mostly because of the obvious - everyone is different. What I look for in a good experience at a restaurant isn't likely to be exactly what anyone else looks for - but someone might have *similar* tastes.
Part of what makes reviews on forums difficult is that for the most part, the reader has NO IDEA what type of person the reviewer is. For instance, my last trips to WDW, it has just been me and my mom in a wheelchair for the majority of our stay there. Sometimes we are joined by my friend who lives much closer to Disney than we do. My mom has had brain surgery and sometimes that effects her speech. All of this factors into my restaurant experience.
The wheelchair gives us an opportunity for more things to go wrong than a party that doesn't include a wheelchair. That server who was so kind and patient to you might have been frustrated when my mom insisted on getting out of the wheelchair to sit in a regular chair.
The server who is impatient and rude with us might be great for a party who is in a rush and knows immediately what they want, for example.
I drink a lot of water - its been my beverage of choice since I was a child. I do get irritated if my glass sits empty for an extended period of time - its not like I can just get up and refill it myself, you know. That's my server's *job*. So if my refills are slow in coming, that has an effect on my opinion of a restaurant. Others who don't drink a lot with a meal would think I'm being picky.
Numerous visits have an effect, too. Some of the restaurants are just less magical and special when you visit them more frequently. The person who finally got into Le Cellier and was wowed by the experience could have had the *exact* same experience someone had on their 5th visit. By the 10th visit it isn't magical, its just what is expected.
And of course, by your 4th or 5th visit, you might now realize that Tutto Italia isn't any better than your favorite local Italian eatery back home, so you skip it in favor of another restaurant. You might read other people (like me) say that they can get food of equal quality at a better price back home, but this might not be true for *you*.
And yet another factor is something as simple as salt.
The cook one night might be heavy handed with the salt - which would be great for someone who likes salty food and horrible for someone who doesn't. This extends to most restaurants - the kitchen staff rotates somewhat and while the recipes are identical, one person's pinch is another person's dash. Sometimes the small variations make the biggest impressions.
I've always wanted to find or create a review site where everyone had comprehensive profiles as I think most reviews are fairly useless for real-life application without knowing a bit more about someone and their expectations of what they are reviewing. This doesn't apply, of course when half a dozen people post that the wait for Ohana was so long and the staff unorganized - I think things like an hour long wait is a more objective statement than "I didn't like the food."
