My Smart Home

I'm currently adding some sensors to my appartment, in order to make it "smart". What exactly am I hoping to gain from this newly found intelligence?

Data gathering, all presented with some nice dashboards and reports
Adaptive lighting inside the living-room: always have the right amount/color of light
Creating a healthy environment, by monitoring temperature/humidity
Security: did I left the window open? Is there unusual activity while on vacation? Or worse, is the place on fire?
I'm gradually planning to put the following sensors inside my home:

Living room:

Motion/light sensor
Humidiy/temperature (Nest thermostat)
Smoke detector
Window sensor
Lifx lights

Smart radiator valve

window sensor

Door sensor
Motion detector
Smoke detector
I'm going to use the following hardware:

Xiaomi Smart Home Gateway: this system uses zigbee for low-power communication, and it is the cheapest platform to use. The gateway starts at 30 USD, and sensors/buttons are available at the price of about 10 USD each. The only disadvantage of this product-family is that it is really aimed at the Chinese market, so the device will yell some chinese instructions at you while trying to pair your sensors. You will also need a chinese powersocket adapter. However this should not pose any problem.
Nest Thermostat: I like the Nest thermostat because it is very clean looking, and has a great user interface.