Okay, the expiration date on food is a false analogy...and that has been already addressed. The food doesn't expire because there is a date; it has a date as a warning to consumers of a natural expiration of biological material (Twinkies, for example, don't have expiration dates).
Whether or not Disney is being "greedy" or not is a bit of a strawman argument. Not all of those who are opposed to the expiration dates are basing their arguments on corporate greed. My comment early in this thread is not based on corporate greed. Corporations are greedy...this is a fundamental aspect of capitalism. My comment was pointing out a change in emphasis for Disney. By eliminating non-expiring tickets and also requiring tickets to be used in the year that they were bought, Disney is potentially hurting long-term loyalty to reap short-term profits.
It is possible that this change occurred because of abuse (people selling unused ticket), but there's other ways to address that abuse (such as requiring the ticket to be assigned to an individual, which they also do. However, look at Debbie's post. Non-expiring tickets caused her to go to WDW more often than she probably would have otherwise. I'm also fairly certain that when she came, she spent money in the parks.
My point is simply this: non-expiring tickets encourage people to return. With ticket expiration dates, people are more likely to buy just the number of days they are sure to need. When the trip is over, there is nothing (except the enjoyment of the experience) to lure them back for another trip. With the exception of DVC (which is a huge commitment for many families) and with the ever-increasing price of the parks, WDW promotes itself more and more as a single-visit vacation.
Of course, WDW isn't hurting for guests. However, if there ever comes a time where Disney wants to encourage more return visits, looking at policies such as ticket expiration dates would be a good place to start.