I suspect that it's a combination of factors, but two in particular:
1. It's what I'd call good comfort food. The cheese soup is good, but heavy. The bread is very tasty, but it's just bread after all. And the food is good but simple. I remarked to my girlfriend that I would have been entirely happy to sit there one night and dip the bread in the soup until I was full. My salmon was good, but it was farm-raised and unspectacular. Here's the point, I think.... people who love comfort food love the place, and report their love back to the board, whereupon people who are looking for a more sophisticated experience go there and are disappointed because that's not what the restaurant is about.
2. Because it's one dining credit, a lot of people on limited budgets go there as the highlight of their trip. It gets a reputation as the best restaurant in Disney because it's better than most of the other one credit restaurants, and for a lot of people without access to fine steakhouses (or in some cases the means to dine at them), it's better than Chili's and Friday's. Other people then go there thinking it's the best restaurant in Disney, when (in my opinion) a much better steakhouse is outside the International Gate in the Yaucht Club, and (from what I understand) another fantastic one down the line at the Dolphin. Said people are consequently disappointed.
Hey, this is just speculation, and maybe I'm wrong, but this is my best guess. Obviously different tastes are at work, as well..... I know a lot of people love Ruth's Chris, for instance, and I think they have tasty steaks (if over-buttered), but they're nothing compared to my favorite steakhouse in the world, Peter Luger. This kind of disagreement is part of talking about restaurants.... anyone may love the place (whether they're used to fine dining or not) or hate the place (again). I just think that in the case of Le Cellier and its wildly differing reviews, this is too simplistic an answer, and other factors are worth considering.
I don't know beyond that. I like comfort food as much as fancy dining, so I enjoyed myself there even if it wasn't my favorite place. As such, I could easily take issue with those who triumph it as an amazing place as well as those who think it's horrible. My best recommendation is nothing new.... make a reservation and try it yourself, free from prejudice-- the only way to make up your own mind!
Just about every WDW restaurant gets mixed reviews here. Two reasons for that: 1) Zillions of folks with different tastes, and 2) Serving zillions of people with different tastes results in mixed outcomes for each dish. 
