Via code inspection, I see that the "server.nnn" configuration key does not support literal IPv6 addresses because the property value is split on ":". In v3.4.3, the problem is in QuorumPeerConfig:
In the current trunk (http://svn.apache.org/viewvc/zookeeper/trunk/src/java/main/org/apache/zookeeper/server/quorum/QuorumPeer.java?view=markup) this code has been refactored into QuorumPeer.QuorumServer, but the bug remains:
This bug probably affects very few users because most will naturally use a hostname rather than a literal IP address. But given that IPv6 addresses are supported for clients via
ZOOKEEPER-667 it seems that server support should be fixed too.