After following Datastax' community edition installation guide to the letter, I was unable to get Cassandra to start up on an i2.xlarge. There were no logs in /var/log/cassandra and editing init.d to try and see stdout came out with nothing. I tried running

sudo /usr/sbin/cassandra

And saw the following error:

Error: Exception thrown by the agent : Local host name unknown: ip-<removed>

So apparently that annoying issue with the hostname not being resolved (which is seen every time you run something with sudo) is causing the issue.

To /etc/hosts, add: ip-x-x-x-x

Where, obviously, you replace x-x-x-x with your private IP. This is obviously a hack and Amazon needs to fix this, but it doesn't seem like they're in a very big rush.