In our environment we noticed that consumers would sometimes hang when started too close to starting the Kafka server. I tracked this down and it seems to be related to some code in rebalance (ZookeeperConsumerConnector.scala). In particular the following code seems problematic:
val topicsMetadata = ClientUtils.fetchTopicMetadata(myTopicThreadIdsMap.keySet,
val partitionsPerTopicMap = new mutable.HashMap[String, Seq[Int]]
The response is never checked for error, so may not actually contain any partition info! Rebalance goes its merry way, but doesn't know about any partitions so never assigns them...