I think the fundamentals, like Bob said (basic mode settings: M, Av, Tv, P, and the green rectangle) are there but it's the variability of combining the more advanced features, Like when to use active d lighting, using various picture controls and the assortment of picture control preferences that are kicking some butt (Mine especially)
I'm also finding that when you add the variability of ASA/ISO versus it's fixed nature with film it really changes the way the image can come out. I know that sounds like a 'duh" but when you are so used to just the 2 factors (aperture, shutter) and you add a third (ISO) now you have a whole new dimension (noise level) that you have to be mindful of.
Instead of only mentally worrying about shutter speed and aperture you now have to be mindful of ISO, shutter speed and aperture because you now have the ISO as a variable factor for each exposure, depending on which mode you are using the camera in.
It took me for ever to realize how the ISO changing (depending upon how you had the camera set) was really playing around with my pictures in a way I didn't like (In auto mode it can go all over the place).
Now I know how to chain that ISO beast back to the setting that I would like so I can concentrate on thinking in "2D" so to speak (Shutter, aperture) instead of "3D" (ISO, aperture & shutter). The fall out is, that for a low light setting, I will be using a tripod more...... but that is just fine with me, I'm a middle aged, slow, old tripod leaning type of a gal!
Then there is the whole active D lighting thing and you have to understand that concept before you can get the most out of your camera (for a particular image that would benefit).
The other thing that can drive a person insane is that only certain features or variables are available in certain modes. You really have to know which modes will accept the changes you want to make and which modes are not available for that feature.
The world marches onward, ever more overwhelming in it's complexity. Remember when phones just used to make phone calls. Now they compare prices and tell you about traffic. It's like that with cameras. I have decided that I want to dive in to the deep end and embrace the technology aspects of today's world. The only trouble is that it's such a deep end that I have no hope of ever becoming proficient in all areas. I have finally realized that it's OK to be a specialist, and to just get a grip on the things that matter most to me. I'll leave the rest up to my 12 year old son. It's his time and it's his world for the most part anyway. All of this technology will become second nature to him and then one day the world will change, or he will decide that he doesn't wish to keep up with it and and the torch will be passed onwards.
Just my 2 cents!
Marlton Mom