Numerous people have reported this in Gitter and I have been able to replicate it on Linux (Ubuntu 14.04). I cannot replicate it on OSX but was able to see the issue with another user (for him, a fresh clone/reinstall fixed it)

Expected behavior

Reaction starts as normal upon a fresh install

Actual Behavior

Nothing renders in the browser. A ton of errors are show in the console starting with:
Uncaught Error: Cannot find module 'bunyan-format/package.json' in check-npm-version.js then a bunch of errors that cascade from there.

Steps to Reproduce the Behavior

On Ubuntu 14.04 just doing a fresh clone, meteor npm install and running ./reaction allowed me to see the behavior.