This is my third JD tractor in about 30 years. I traded in my JD 400 because I couldn't find parts any more. Then several years ago I upgraded to a JD X728 (4-wheel steering). The X729 (4-wheel steering and 4-wheel drive) was not available when I purchased the X728.
The reason I "upgraded" was that while snow blowing, even with chains on the rear wheels, I could slip off the paved driveway and get stuck. The X729 solved that problem and has the added benefit of (at least in my case) not needing chains on the tires which is much easier on my blacktop driveway.
The drive train on this tractor is Sherman-Tank-like but I'm always surprised at John Deere's lack of engineering on the little stuff. I had a part fail on the X728 (right at the end of the warranty period) that interfered with lowering the attached implement. It was a small and inexpensive part, but it looked under-engineered.
On both X728 and X729, the levers that control the attached implements feel too thin and flexible. It seems that another dollar invested would have made them feel more solid.
The only other annoying design flaw is the cup holder. This is just a lack of thinking (caring) on John Deere's part. I noticed this right away on the X728. The built in cup holder has 3 or 4 tiny holes drilled into the bottom for draining fluid (rain).
The holes are so tiny that they get clogged up all the time just from normal usage. I thought they would have fixed this problem 3 or 4 years down the road with the X729 but nooooooo...same clogged up holes.