That first candidate is mostly a developers release with full adodb support, and many new features essentially for testing their ability to be stable in final 1.8.
New features include mapserver feature, articles, forums, newsletter and wiki enhancement, some new plugins and modules,
Additionally TikiWiki 1.8 supports now other databases like Postgres, Oracle, Sybase and SQLite (built in PHP 5.0!).
Read more in ReleaseProcess18.