The price — $US 850 vs. $US 420 for the LX3 (B&H, Nov 2008).
A longer warranty (2-3 years, depending on country).
Better re-sale value (? wishful thinking?…).
The D-LUX 4 has a nicer styling and finish. It has also omitted the LX3's built-in hand grip. If you want a grip then you have to buy one as a Leica accessory.
Leica claim they use the "pick of the litter" when making the D-LUX 4. This doesn't mean the rest of the LX3's are duds, but rather that there is a greater likelihood of the D-LUX 4 lens, mechanics and electronics being in the middle of the Q-C sweet-spot — see the Jan 2009 discussion at <L-camera-forum.com: #75735>.
RAW files created by the D-LUX 4 use a different filename suffix: ".RWL" as opposed to Panasonic's ".RW2". (There are also internal EXIF tag differences, but otherwise the actual CCD image data is byte-for-byte identical.)
The included RAW developer software for the D-LUX 4 is Capture One v4, whereas for the LX3 it is Silkypix. This is a bigger difference than you think, because Silkypix can fully correct for barrel-distortion and chromatic aberration automatically, whereas C1 only does basic barrel correction. To get more robust correction you have to pay extra for "C1 Pro" (see also the discussion about RAW shooting below).
The image firmware for both cameras is completely different — many claim JPEG colours created by the D-LUX 4 appear to be more "natural" and "Leica-like".
The (much) cheaper LX3 is difficult to buy as few stores ever have any in stock — see this April 2009 blog entry by Mike Johnston.
