Economically speaking, it's highly unlikely that the dining plan itself is the cause of lower quality.
First, they don't know if you're on the dining plan until the end of the meal, so it's unlikely that they lower quality on dining plan meals (as some have suggested). And even if someone is on a
free dining plan, they won't know that either, or at least, not until the end of the meal. Not a fan of these conspiracy theories.
Second, the dining plan is still paid for (usually), and while there is a small discount if you get the most expensive options, what you're really paying for is the convenience of pre-paying for a meal. Most guests to not get full or over-value for these plans. If Disney wants to increase profitability on these plans, there are better ways to do it, like removing features or requiring more credits ... both of which they've done for 2011.
Third, even in the case of a free dining promotion, you have no way to know who's on that promotion and who isn't when they order. Are you going to risk lowering the quality of food even for paying customers? That's not normally a risk you would take. Free Dining is offered to get guests into the restaurant in the off season to keep the restaurants fuller and (counter-intuitively) reduce costs.*
Disney could be intentionally reducing quality across the board to try to increase profitability at the restaurants, but it's unlikely to be related to the dining plan itself. However, I believe this scenario is entirely unlikely as well. Food quality can be a health issue and it's something Disney takes very seriously. Given that there seems to be as many positive reviews as negative, it's more likely just general variations in quality, amplified by the Internet.
Personally, I think the OP likely just had an epically bad night, a perfect storm of service and maintenance failures. For management to have done nothing to fix the situation is tragic (because it leads to posts like these), but I'm not sure why the OP also didn't INSIST on being compensated. If I had had this happen, I would have insisted that the entire table's meals were comped.
Imagine the following ...
1. There's a new chef, not quite up to speed yet, so a handful of meals come out not done correctly.
2. Maintenance missed the loose chair.
3. Dishwasher didn't thoroughly clean the class.
4. Server also missed the dirty glass.
Any one of those things might be something you'd handle and go on about your meal without issue. Start combining them, rare as it may be, and you'd have a pretty bad night. Have all of them happen at the same table? Yeah, that's a disastrous experience.
(*Studies have shown that busier restaurants tend to have higher quality, because of a variety of factors, such as the Hawthorne Effect, less "slack" time for the staff, and generally more attention being paid to overall quality than in slow times. I've also observed the same anecdotally. That said, there will still be issues. Also, a higher number of customers per hour reduces the cost of producing each meal (staffing is amortized across the meals produced), so keeping the restaurants full reduces costs for those meals. Yes, I was a business major in college.

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