Q:

Yosemite won't boot any more after attempting to enable FileVault and TRIM

I installed Yosemite yesterday and it was working fine.
Now whenever it tries to boot it runs Safe Mode and fails 1/3 of the way through.
From what I can remember I went to enable FileVault and backed out of it as I didn't enter my correct iCloud information, and didn't want to go down the key generation route.
I also tried to enable TRIM.
This morning I restarted my machine after installing Kies and it wouldn't boot. The error I'm getting is this:
Oct 26 11:01:33 localhost Unknown[358]: Keyboard Layouts: duplicate keyboard layout identifier -16899.
Oct 26 11:01:33 localhost Unknown[358]: Keyboard Layouts: keyboard layout identifier -16899 has been replaced with -28673.
Oct 26 11:01:33 localhost Unknown[358]: Keyboard Layouts: duplicate keyboard layout identifier -16900.
Oct 26 11:01:33 localhost Unknown[358]: Keyboard Layouts: keyboard layout identifier -16900 has been replaced with -28674.
Oct 26 11:01:34 localhost Unknown[358]: Setting boot completed.
Oct 26 11:01:34 localhost Language Chooser[357]: Could not write to /var/log/CDIS.custom
Oct 26 11:01:34 localhost OSIESpringboard[372]: Can not connect to /var/run/systemkeychaincheck.socket: No such file or directory
Oct 26 11:01:34 localhost OSIESpringboard[372]: Failed to unlock login keychain.
Oct 26 11:01:34 localhost Unknown[358]: 2014-10-26 11:01:34.864 OS X 
Utilities[372:303] *** WARNING: -[NSImage compositeToPoint:fromRect:operation:] is deprecated in MacOSX 10.8 and later. Please use -[NSImage drawAtPoint:fromRect:operation:fraction:] instead.
Oct 26 11:01:35 localhost Unknown[358]: Keyboard Layouts: duplicate keyboard layout identifier -16899.
Oct 26 11:01:35 localhost Unknown[358]: Keyboard Layouts: keyboard layout identifier -16899 has been replaced with -28673.
Oct 26 11:01:35 localhost Unknown[358]: Keyboard Layouts: duplicate keyboard layout identifier -16900.
Oct 26 11:01:35 localhost Unknown[358]: Keyboard Layouts: keyboard layout identifier -16900 has been replaced with -28674.
Oct 26 11:01:40 localhost Unknown[358]: 2014-10-26 11:01:40.167 OS X Utilities[372:303] *** WARNING: Method userSpaceScaleFactor in class NSWindow is deprecated on 10.7 and later. It should not be used in new applications. Use convertRectToBacking: instead.

What could be causing this error?
I've tried two things:

Installing mavericks from the recovery drive - this fails.
Disabling trim from the command line.

Neither of these work, is there something else causing this issue?
Edit: verbose mode stops at "still waiting for root device"

A:

Yosemite apparently uses a new security feature called kext signing. This will prevent the old method of TRIM enabling. See the Trim Enabler FAQ, The question Recovering from stop sign on boot screen might help you:

Recovering from stop sign on boot screen
For those who are stuck on the grey boot screen, here’s how you get
  back into OS X: 
Step 1: Boot recovery mode by holding Cmd+R during boot
Step 2: Is your volume encrypted (FileVault)? If so, first open Disk Utility from the menu bar and unlock/mount your drive. [see here for the procedure]
Step 3: Open the Terminal from the menu bar
Step 4: Run this command: nvram boot-args
Does it say kext-dev-mode=1? if so, you can skip to Step 6. If it
  says error getting variable, continue with these steps:
Step 5: Run this command: nvram boot-args=kext-dev-mode=1
Step 6: Reboot back in to Recovery Mode again
Step 7: Run these commands, replacing Your Disk Name with the name of your Mac disk
  (partition). You can type ls /Volumes to get a list of volumes. Note
  the quotes around the disk path and that there should be no / before
  System.
cd "/Volumes/Your Disk Name"
touch System/Library/Extensions
kextcache -u "/Volumes/Your Disk name"
Step 8: Wait until it finishes (can take as long as 5-10 minutes,
  don’t abort it) and reboot. You should now be back in OS X with Trim enabled.
The commands are not working!
The commands will work if they are entered correctly. However, if you are not familiar with the Terminal, it can be hard to follow the steps above. If can’t get the Terminal commands to work, or want an easier option, you can also use the Recovery Mode to reinstall OS X. Reinstalling OS X will only reinstall the system files, leaving your files and preferences untouched.

Please check the original source for updates!
EDIT:
See my answer here: Since the release of 10.10.4, Apple now provides a new tool called trimforce, allowing users to activate TRIM also on unsupported disks. No more dangerous experiments with third party software necessary.
/EDIT

