I've been carrying an SLR camera to WDW since 1999.
Carrying the extra size and weight does take some adjustment, but once you figure out your preferred method, you'll never even consider leaving the camera home.
The way I figure it, if you're going to spend $1000 or more on camera, lenses, batteries, memory, and accessories, why on earth would you even consider not taking it with you to the number one photo op of the year - your WDW vacation? It's like buying a new wardrobe and leaving it in the closet, while you go to work in old sweats and slippers.
The best advice I can give for handling the size and weight all day is to pare it down. Don't be tempted to take all of your lenses and all of your memory and your tripod and cleaning kit and filters and a filter wrench and the instruction manuals and the kitchen sink and a spare blanket and a picnic basket...
Pare it down to camera, one good walkaround lens, a few spare memory cards, a spare battery, and a lens cloth.
Leave the camera on all the time. Don't put the lens cap on - you can't take pics that way! Don't think of taking a picture as a major event that you have to stop, thing about, prepare for, and set up. Taking a picture is like breathing - if you practice, you can do it on the move, without thinking, and miss far fewer photo ops. But taking a pic is also not something that has to interrupt your day and steal time away from the fun; just keep moving, and keep shooting.
Eventually, the camera won't feel like a load, it'll just be something you wear, like your shirt or sunglasses.