Reliability is a huge, huge deal. Back in 2009 we were dealing with constant nozzle jams, delamination from the print bed, print errors, drifting, ooze, extruder slip, and all sorts of other problems. You had to keep a constant eye on the printer, and manually tweak the G-code generation parameters to get good results. It's come a long way and still going.
There’s also constant improvements to the algorithms; what can be printed in the first place. To name one, it used to be that anything above a 25 degree overhang would cause problems…
These days 60 degrees isn’t too bad, and there are experimental algorithms that can handle 90. Support algorithms have also improved dramatically lately, so we’re pretty close to the point where any and all geometries will just work.
Several of the features in the MK4 are for quality (but also help speed). For example, VFA steppers are purely for quality improvement.