There has been some good descriptions so far of the ones we've encountered over the years, but let me summarize...
1) Some folks go to Disney expecting an amusement park, and it isn't. We always try to describe it to friends as an "amazement" park. Fun, colorful, but not necessarily "white knuckle".
2) Some just don't realize that any trip to WDW needs planning. You don't have to schedule every second, but there is simply TOO MUCH to do it all in a normal 5 day type trip. Plus, they go in the summer (convenient) or Christmas ("we heard it's beautiful") and it ruins their view of WDW. I don't blame them, my wife and kids will have to tie me up and drag me if they want to ever go in the summer again... No Way!
3) One more reason I run into which really wasn't mentioned, but seems to be included in every one who wants to argue angainst going to WDW is money! "the food's ridiculous, those tickets are outrageous, it's way too much money to stay onsite, so we got a nice Motel 6 20 miles down the road".... Face it, some people want to a Disney vacation on Yellowstone Park budget and that just isn't going to happen. No slam on Yellowstone BTW, just a point of reference.
As much as we enjoyed our very first "low budget" trip to DLC, we didn't love the whole Disney experience nearly as much as we do now. The more we do it, the more we learn, the more you great people on this forum share your "wisdom", the more amazing the Disney Magic get. If we could just get the ones who hated it to let us help them plan one, maybe they would all convert!