I tried giving Cassandra 2.0.0 a whirl tonight by installing the dsc20 package from the DataStax Community repository but found that Cassandra would not start due to incompatible class files. The error you’ll see in output.log looks something like this:
CassandraDaemon : Unsupported major.minor version 51.0
If you’ve done much in Java you’ll probably recognize that this is a version inconsistency and, sure enough, you need Java 7. The dsc package doesn’t seem to specify this (and the opscenter-free package actually depends on java-6-jre-headless if you apt-cache show the package.
To solve this you’re going to want to install Java 7 and update your configuration to set it as the default.
The update-alternatives command will prompt you to choose which Java you’d like to use. After all this, give starting Cassandra another go and you should be all set.