The touch screen is most useful for quick focusing. I use center for AF Area. But if my subject happens to be right or left of my frame, i simply tap screen on the subject and the AF will be on the subject. Makes framing/focusing very fast.
For PASM mode, the button in center of control wheel (soft key C as Sony calls it) is the mode key. So say you're at Av (Aperture Priority) and wants to switch to Tv, all you have to do is to press soft key c, and turn the control wheel by one click to S (Shutter Priority). So it's just one step more than traditional DSLR. I came from Canon DSLR, but find switching to Nex 5N easy and intuitive. In any case, most people have preferred mode, say Aperture Priority and seldom change it so don't really miss the lack of direct buttons.
Say you're at A mode in Nex 5N, just turn the control wheel to change aperture. If in S mode, just turn the control wheel to change shutter speed, so it's all quite intuitive. I usually use M mode. Press "down" on the control wheel toggles shutter speed and aperture.
