Before our extensive remodel 2-3 years ago, I had similar questions. I spent a lot of time reading the various forums, which have a fantastic wealth of information disseminated by very knowledgeable folks. After a while, it came down to me figuring out the answers to 2 questions, which you are asking as well:
1) What do I want to automate/distribute/control ?
and most importantly,
2) Why ?
The "why" part, for me, was for aesthetic reasons, convenience/ease of use, and features. The "what" proved to be a little more difficult, as I started with a list of everything you have in your initial post. I quickly saw that I had to do it in phases, as all my money, and more, was going into the remodel. The key to it was getting the proper wiring in place when the walls were down.
What I wanted to do was lighting, security, whole house audio, and possibly HVAC.
We have lived in the remodeled house for just over a year now, and I have barely gotten into the "home automation" part! I must say, I don't have a home automation controller/system yet, but the sub-parts are controllable.
1) WIRING: I spent a lot of time reading on what wires to run where, and got that in place. I got a security company to run the wires for a whole house alarm system, and I chose Lutron for lighting, so had the Lutron person run the wires for the lighting system (HomeWorks4). I ran a whole bunch of cat6, RG6 and speaker wire so I could distribute audio/video/data at a later point.
2) LIGHTING: This was at the top of my list. Our house is not huge (about 2700+ sq ft), but I felt that the convenience a system like Lutron would provide was worth the money. Some examples of what I can do with the system:
* Hook it up to the security system so all lights can come on inside, and outside lights can flash in case of an event.
* The guest room doesn't have an attached bath - so there is a button in there that will turn on the lights in the foyer, living room (at half brightness) and guest bath to illuminate a "path" to the bathroom, so when my mom visits, she doesn't have to stumble in the dark, or worse yet, step on my kids toys that are strewn about.
* When we go out of town, I can set a "vacation" mode, so lights turn on/off in our house just like when we are at home.
* I haven't created "scenes" yet, because we wanted to live with the system in our house for a little while to figure out what we really wanted. But that is another convenience.
* When I am in bed upstairs, I can see that I left the light on in the kitchen, and turn it off without getting out of bed.
* I have keypads instead of multiple light switches in a row - much more aesthetically pleasing.
To answer your question, this part - Lighting - I find truly useful, and I am glad I invested the money in it.
3) SECURITY: I have wires run to every door and window. I have wires for motion sensors in a few rooms. I haven't done anything more with it yet (which is why, I say "I can hook up the lighting to the security system" :-)). I still think this is truly useful to have as part of a whole house control system.
4) WHOLE HOUSE AUDIO: I ran speaker wires everywhere. I did not run any cat6 wires to possible keypad locations, or loop speaker wire through there.
On really thinking about how I wanted to control different aspects of my house, including the music, one thing I did not want was touchscreens/keypads/volume controls/etc fixed in the wall (exceptions would be light switches, and 3 security keypads). I would rather use a good remote, iPhone, iPad, my laptop, etc. That was my personal preference, which is reflected in the wires I ran.
Finally, I just bought a Sonos bundle, and will be adding to it. I have someone coming in to install a 5.1 system in the family room (our primary TV viewing room), and they are also going to install speakers in the kitchen, and patio - I already have some speakers in the living room. I am planning on getting a Sonos S5 for our master bedroom.
I can't really say if I will find it truly useful without using it, but I am sure in a couple of weeks I will be saying it, given the number of occassions I had wished we had it.
5) WHOLE HOUSE VIDEO: To me, this means centralizing sources and controlling them from anywhere in the house. This was not on my list at all - simply a personal preference. I found it easier to have a BluRay player and a box for cable/satellite/etc at the TV location. I only have 2 TVs in the house, so this approach worked fine, and I did not have to worry about matrix switches and ways to control it. I do have enough wires run to a central location, so I could do this later if I really wanted to for some reason.
6) HVAC: I ran cat6 wires to each thermostat location (I have 3 locations, 2 units, 3 zones). Can be controlled in the future if I want to.
7) IRRIGATION: We don't have a huge back yard, so this is not on the list to bring under the umbrella of "automation/control".
8) CAMERAS: I ran some cat6 and RG6 wires to possible camera locations, but I am not sure if I am ever going to use them.
All this and I haven't even gotten to the control part yet.The reason for getting a control system for me is this - right now, I can control the lighting through my iPhone, and computer (apart from the switches themselves). Sonos has it's own really nice remote. To control the thermostats, I have to walk to 3 different locations on two different floors (although I don't do it very often, if I have a control system, I might as well put this on there and enjoy the benefits). For my home theater I have a remote for the TV, BluRay player, receiver, UVerse (sure, these can be consolidated into one remote).
It would be great to get a single, *consistent*, easy to use interface for all these different systems, so everyone in the house can control anything easily. I am still contemplating the price to convenience ratio for this. There is always the cool factor of hitting the "movie" button, and having the right components come on with the right settings, the shades coming down and the lights adjusting to the perfect setting!
I apologize for this long-winded post that probably doesn't answer your questions about the control of these sub-systems themselves, but it's just another perspective.