Have owned this car for a month after having bought used off eBay for under $9000. 9 years old with 95K miles and it's still tight, rattle-free, and wonderful to drive. The leather is in better shape than that of most 3-year old cars. It makes you wonder why anyone would ever buy a newer, more pedestrian sedan from Hyundai, Ford, or even Honda. More comfortable seats than Benz or BMW. Aside from a defective antenna (a $25 repair you can do yourself in 5 minutes) everything is perfect. The independent mechanic who inspected it just whistled and said, this will run forever. Even if I have to spend $1K-$2k in repairs later, it will be a bargain. I already want a 2000 LS 400!
Decent Nakamichi stereo only bettered by newer Mark Levinsons. Very comfortable seats with good lumbar support. Adjustable air suspension. V8 power with a smooth transmission. Decent gas mileage. Classic, timeless design.
Nothing! More gadgets would only make it less reliable. And the 1998-2000 series LS 400 has changed everything that needed improving. I think they actually went backwards with the LS 430.