1) a simplification of the tax rules - mainly by reducing the number of exceptions and tax deduction rules,
2) filing the tax forms digitally - they started with desktop applications, and now the main focus is on a web application, and
3) pre-entering the information the DTA already has on you, so you just need to correct and add missing information instead of entering everything from scratch - and they collect a lot of data of you yearly.
This improvement happened over the last 15 years, and the time required to filing tax returns was reduced from many hours (mainly to gather all the information, and writing it on a paper form) to like 30 minutes. That is, if your administration is up to date. Filing the tax forms is my yearly reminder to update my administration...
BTW, that slogan is stated more succinctly in Dutch: "Leuker kunnen we het niet maken. Wel makkelijker."