Oracle originally planned to have its JDK 8 Milestone 6 preview be feature-complete, but instead they ended up delaying over a dozen Java Enhancement Proposals to the next milestone. Most of the delayed features revolve around Project Lambda, an effort to add closures to the language. Project Lambda has already been partially integrated into Java, but some open items remain.
Right now Oracle developers are still determining how much of an extension will be needed into the release schedule for this additional work. The plan up to now was having the general availability of the Java Development Kit 8 in early September.
More details can be found on the jdk8-dev mailing list.