After installing Yeoman and some generators, I kept seeing a bunch of lines beginning with npm WARN unmet dependency. Something was amiss. A quick Google search brought me to a GitHub issue from a user who was having the same problem. The fix, which the OP discovered himself (herself?), is to run this line in the Terminal:

ulimit -n 10000

Once I did that, I just had to install Yeoman (npm install -g yo) and my preferred generators again, and my troubles were gone.