I have some experience with the older JD LT series and L series.
I own an L111 (20hp Briggs V-twin, hydrostatic trans) that I have been using for the past 10+ years. It has about 120 hours on it, runs well, and is still in good shape overall. The only things I've had to replace on it are mostly routine maintenance items (a couple of batteries, fuel filter, air filter & oil filter). The only non-routine thing I've replaced is the rubber latching strap for the bagging attachment.
My father has an LT-155 (15hp Kohler, hydrostatic trans) that is about 15 years old. His lawn tractor has also given good service. There are only a couple things I know that have failed other than routine stuff; His steering sector & gear wore to a point where the tractor would not turn properly. I put a new steering shaft / sector on for him about a year ago. He also has had to replace the rubber latching strap on his bagging attachment, as well as the grass bags themselves. I also have replaced the belt for his twin blade mowing deck.
In my opinion, there is not as much difference in quality between the two lines as a lot of people suggest. Yes the LT series (or current 300 series) is in general somewhat higher quality, but when I bought my machine from a JD Dealer, I looked at the LT150 / LT160 and chose the L111 over the LT models. The LT series did not justify the price difference in my opinion. I also liked the smoothness of the twin cylinder engine in the L111 over the single cylinder engine in the LT 150/160. The dealer was more than happy to sell me an L-series tractor. I suspect most JD dealers today would gladly sell a D-series tractor as well.