We plan to switch JavaFX to using Native Look and Feel by default in Marina. This is being done for two primary reasons: 1) to improve startup performance; 2) rendering quality on Mac and on pre-JDK-6u10 Windows.

Dmitri writes the following:

Reasons for selecting platform L&F by default: Nimbus is only available on 6u10+ (which currently means windows only). On Mac we would select Metal, and since the default java is java5 on Mac, it looks very bad (Metal doesn't look good on Java5, especially for non-English languages, because of font rendering quality).