There are two main regressions that I've identified when setting the version explicitly to the empty string, that worked as expected in 0.7, but do not in 0.8:
Either "unspecified" or "" is added to a jar name depending upon where it is being used, but it's all related to the war plugin

[project1/build.gradle]
usePlugin 'java'
version = '' // This is the empty string

jar file => project1.jar (this is still the same as 0.7, which is good)

[project2/build.gradle]
usePlugin 'war'
version = '' // This is the empty string
jar.enabled = true
dependencies

war file => project2.war (this is still the same as 0.7, which is good)
jar file => project2-unspecified.jar (it used to be project2.jar in gradle 0.7, and using just java plugin implies it still should be)

project2.war contains => project1-.jar (it used to contain project1.jar in gradle 0.7, without the unexpected "-")

Explicitly setting version = null ... or never setting it does result in consistent behavior, although it means everything is
project1-unspecified.jar
project2-unspecified.jar
project2-unspecified.war

Appendix and classifier can still both be set to the empty string and they get ignored as expected, but version is now behaving differently.