1) If it rains, you are screwed. Sure there are weatherproof cameras (my Canon T1i does not qualify) and there are methods and equipment to help protect your gear, but when you are an ultralight hiker lugging a DSLR, there has to be limits! Beyond equipment issues, when you are hiking through the rain your general state of mind is to press on, not stop and enjoy getting wet while shooting. In other words, it is a mental block as well.
2) If you are doing big mileage, it is all about the hike. In learning how serious photography and hiking can co-exist, doing 15 plus miles of hiking nudges the photography out of the picture. I would say 10 or less miles leaves the time and energy to shoot, any more than that and hiking dominates the day.
3) Unless you are in a wildlife mecca like Denali or Yellowstone, leave your telephoto lens at home. My main focus was landscapes. Just stick to the wide angle lens and miss the other stuff. The extra weight of additional lenses is not worth it.
4) Need to see in black and white better. I continue to try and picture what subjects would be best captured in B&W.
5) A DSLR is heavy. Any fantasies I had of bringing my T1i on the Tahoe Rim Trail this summer (165 miles around Lake Tahoe) were dashed when I realized hiking 95 miles in Scotland with a DSLR was a mistake, or a learning experience if you want to put a positive spin on it. Panasonic LX3 is my long distance hiking partner. Lessons learned!