'cause the actual changes that break between npm 1 and 2 are very small. (the changes to npm run, mostly, and the cache.) so it's Very Strange(tm) modules that break, and the new run behavior is much more useful
- npm team member
It's not really anything against npm itself and if the node team doesn't do this with high frequency then it won't be an issue. The 5.x coming so soon after 4.x was just jarring and everyone is speculating how frequently they're going to make breaking changes.
I realise this makes the package.json Node-specific, which might be something you want to avoid.
EDIT: to part-answer my own question, package.json has the "engines" field. Not sure if that means it automatically installs old versions in older versions of Node, though.
Basically, you get an error but it won't give you the proper version. If that has changed I'd love to know, however (it most certainly should install the latest version for your environment and give you a warning that it's not the latest. In my opinion anyway).