Hard Mode
If you allow all 6 Saronite Vapors to spawn without killing them, and while Vezax is still alive, a Saronite Animus will spawn.
The Animus will make Vezax immune (well, 99%) to damage (but he will not cast Searing Flames). The spawning of the Animus is considered the beginning of the hard mode.
Shadow Crashes will still exist however.
Also, shadow crashes are much, much more frequent. While the hard mode is easier to get to in terms of clouds spawning and HP on the general, ranged DPS and healers will have to move around much more frequently.
I am not sure if the Animus requires a separate tank or not. I know he is not tauntable etc, so misdirects are important if the tank doesn't snag it (it spawns where Vezax initially stands).
What I am actually more concerned about is this: we do not have this achievement:
http://www.wowhead.com/?achievement=2996HM Vezax guarantees an instagib if you are hit with a Shadow Crash.