I personally prefer the OM602 (NA) to the OM617 (turbo) or OM616, mainly for the belt design, hydraulic valves, and a reliable Denso AC compressor. Increased fuel economy also helps. OM617 certainly is better for changing glow plugs.

As for things I am fed up with, the M103 is on the top of the list. Poor belt design, poor tensioner design, poor amount of space to work on the front, expensive yet simple ignition system, CIS, and overheating issues.

I plan to fix most of that when I install a 3.0 M103 into the car later in the year with Megasquirt 3, a better tensioner design, and an air-to-oil cooler. Still have to deal with limited space, unfortunately.

__________________
Current: 1975 450SEL, 83 300D, 88 Yugo GVX, 90 300D OM603 swap, 91 F150 4.6 4v swap, 93 190E Sportline LE 3.0L M104 swap, 93 190E Sportline LE Megasquirt, 03 Sprinter, 06 E500 4Matic wagon.