I just ran into this unmet dependencies problem while trying to do an "apt-get upgrade", but also had the additional issue of /boot having run out of space.
Some errors which provide clues that your /boot is out of space:
gzip: stdout: No space left on device
No apport report written because the error message indicates a disk full error
This meant that apt-get was helpless, with "sudo apt-get install -f" and "sudo apt-get autoremove" failing to work.
In the end, the link in Justin's answer provided the solution:
1) Check how many old linux-generic kernels you've got installed:
find /boot -type f -regex "^.*-generic"
2) If more than 2, manually clean out the old ones
sudo find /boot -type f -regex "^.*XX-generic" -delete
Replacing XX with the number of the old kernel you wish to remove (e.g. find /boot -type f -regex "^.*24-generic" -delete) Make sure you leave the latest two kernels (highest two numbers).
3) Now you've got space to fix the unmet dependencies:
sudo apt-get -f install