General purpose distributions including Debian have used sensible defaults for cpufreq and cpuidle for a long time now. Recognizing that mobile computing is very important, and has been for a long time. You do not need to enable these two features.
Manually capping your cpu frequency may reduce power efficiency. It is not particularly recommended that you need to fiddle with the the settings of cpufreq and cpuidle, unless you have a problem.
You can observe that dynamic frequency scaling and idle states are used by running
powertop. This is also a convenient way to view battery drain and observe that it varies depending on the load you put on the computer. (And your backlight brightness, and ...)
Power usage also depends on drivers that know how to fully control the hardware, and that know what specific configurations are safe to enable on your machine. This has become more complex and important over time; it is probably the most common reason for a Linux install not reaching the full potential of the hardware.
acpid is old; it is not really used on Debian Jessie by default. Some of its functionality is replaced by systemd. E.g. systemd-logind can now handle the suspend button - again this includes a sensible default, you only need to change it if you have a problem.
I don't know how much benefit it's reasonable to expect from hard disk spin-down for modern usage. I do believe Windows makes use of it. Currently I avoid it due to the possibility of "green" drives damaging themselves.