Another one inspired by a passing comment above: lightweight bogie design, one of the largest single masses for any vehicle.
Hot axle box detection (HABD), many crashworthiness improvements including anti-climb and far more driver protection than in old units where the driver's cab was the crumple zone itself. Great improvements to bogie and coupling designs that dramatically reduct the chances of vehicles whithin a unit jacknifing in a collision. Most of these have come from better specification rather than pure innovation.
Wheelslip and wheel slide prevention (WSP) means far fewer trips to the wheel lathe than ever, especially in autumn. What was known as SEPEX has also made its way into distributed traction as well meaning that one slippery spot doesn't flatten a tyre.
Flexible vehicle body designs (the Hitachi AT series, Bombardier Aventra, Siemens Desiro City et. al) alongside "raft" based components mean that one size fits nearly all tenders - door layout, vehicle length, engines/transformers/traction motors are all wildly interchangeable now where an entirely new design might have been required before. Just look at the 345s: in some places they have their traction inverters located on the carriage one-over from where the motors are, because it makes the best use of underfloor space.
This isn't limited to multiple units either - the class 58 borrowed some fantastic modularity and maintenance accesibility from the US way of doing things, but even the 73s were pretty good to take apart and put back together again, hence the 73/9 conversions.
