I've been an SLR guy since 1999. I carry the camera on a strap around my neck and shoulder.
I carry my Canon Speedlight flash in my pocket (I always wear pants with deep pockets to WDW).
When I was shooting film, I carried 3 spare rolls in my pocket. Now that I shoot digital, I carry a spare memory card instead.
My old camera used 4 AA batteries, and I carried a spare set in my pocket in a small wallet with the spare memory card. My new Digital Rebel takes a specialty battery, and I have no spare, so instead I will be carrying a polarizing filter in the wallet with my spare memory card.
I have never carried a spare lens. I started with a 28-80mm, which didn't have enough zoom, so after 1 trip to WDW I bought a 28-200, which had plenty of zoom. I currently have only the 18-55 kit lens that came with my Digital Rebel (which is equal to a 28-80 on a film SLR), and next year some time I will buy a Sigma 18-135, which is equal to a 28-200 on a film SLR.
Bags are a pain. But if you plan a little, you can get along just fine without one, and still carry the basic equipment that you need to get great pics.
Just remember, it's not the plane, it's the pilot.