I can't run projects alongside Kafka that use zookeeper 3.4 jars. 3.4 has been out for 2.5 years and seems to be ready for adoption.
In particular Apache Storm will upgrade to Zookeeper 3.4.x in their next 0.9.2 release. I can't run both versions in my tests at the same time.
The only compile problem I saw was in EmbeddedZookeeper.scala