This is mainly a camera support release this time round (Canon 450D, Nikon D60 etc).
New cameras: Canon 450D, Fuji S100FS, Nikon D60, Olympus SP-570 UZ, Pentax K20D, K200d, Sony A200, A300 and A350.
The LR release has also improved compatibility with Leopard print drivers.
All cameras have been updated to ACR 4.4 which offers better rendering at the ends of the temp/tint scales.
Grayscale conversion noise has been greatly improved in this release.
Those using export plugins may need to download updated versions of the plugins.
EDIT: Lightroom 1.4 was withdrawn a couple of days later due to 3 unforeseen bugs. 1.4.1 should be re-released shortly.