The Mac version of IntelliJ IDEA 7 is built with JDK 1.5. This only affects performance - there are no features missing.
As for Leopard, it's a mixed experience. On one hand, the Leopard version of Java fixes a few critical issues which can cause hangs of IntelliJ IDEA 7 when running under Tiger. On the other hand, there are some UI glitches specific to Leopard. Some of those glitches will be fixed in IntelliJ IDEA 7.0.2, while others seem to be Apple bugs.
Users have also reported that Java applications including IntelliJ IDEA don't work well with the Spaces feature on Leopard.