I've recently upgraded to Ubuntu 14.04 64 bit (in the past I've used only the 32bit version) from the past LTS. I'm quite happy with it but I've found a few glitches.

The last problem I've faced is

Fatal error: Call to undefined function mcrypt_module_open() in lib/Varien/Crypt/Mcrypt.php

Obviously php5-mcrypt is'n installed by default so:

sudo apt-get install php5-mcryp

seemed the natural solution ... but .. still you have to manually enable it

sudo php5enmod mcrypt