There are many, many opinions of this all over the map. Unsurprisingly, many "ultra-Orthodox" Jews apply the term to many things today where many "Centrist Orthodox" Jews feel it doesn't apply.
The Aruch HaShulchan wrote that there is no prohibition of mesirah with regards to a country with fair laws, "such as England or the Czar's Russia." Some believe he wrote that entire phrase to placate the censors; Rabbi Yitzchak Breitowitz believes he wrote "England" honestly, then paused and realized what the Czar would think -- so he added in "or the Czar."
More broadly, Rav Wozner's opinion is that in cases where dina demalchusa dina applies -- i.e. where Judaism acknowledges the local government's right to govern -- then it is also fully appropriate for everyone in that society to uphold such laws, and there is no prohibition of mesirah.