Straight from Microsoft Blog:
Here’s the key difference for Windows 8: as in Windows 7, we close the
user sessions, but instead of closing the kernel session, we hibernate
it. Compared to a full hibernate, which includes a lot of memory pages
in use by apps, session 0 hibernation data is much smaller, which
takes substantially less time to write to disk. If you’re not familiar
with hibernation, we’re effectively saving the system state and memory
contents to a file on disk (hiberfil.sys) and then reading that back
in on resume and restoring contents back to memory. Using this
technique with boot gives us a significant advantage for boot times,
since reading the hiberfile in and reinitializing drivers is much
faster on most systems (30-70% faster on most systems we’ve tested).
Use of multiple cores: It’s faster because resuming the hibernated system session is
comparatively less work than doing a full system initialization, but
it’s also faster because we added a new multi-phase resume capability,
which is able to use all of the cores in a multi-core system in
parallel, to split the work of reading from the hiberfile and
decompressing the contents. For those of you who prefer hibernating,
this also results in faster resumes from hibernate as well.
Shutdown+TurnOn≡ColdBoot: Another important thing to note about Windows 8’s fast startup mode is
that, while we don’t do a full “Plug & Play” enumeration of all
drivers, we still do initialize drivers in this mode. Those of you who
like to cold boot in order to “freshen up” drivers and devices will be
glad to know that is still effective in this new mode, even if not an
identical process to a cold boot.
And this point is worth mentioning regarding your Edit2 where shutdown+trunOn didn't fix your problem, but Restart did:
When to use Restart specifically: Of course, there are times where you may want to perform a complete
shutdown – for example, if you’re opening the system to add or change
some hardware. We have an option in the UI to revert back to the
Windows 7 shutdown/cold boot behavior, or since that’s likely a fairly
infrequent thing, you can use the new /full switch on shutdown.exe.
From a cmd prompt, run: shutdown /s /full / t 0 to invoke an
immediate full shutdown. Also, choosing Restart from the UI will do a
full shutdown, followed by a cold boot.
More extended info, visit: Delivering fast boot times in Windows 8