This is one of those posts that's likely to become obsolete very quickly as the install process for Ionic changes, but if it helps someone else resolve the (minor) install issues I ran into today, then it's worth doing.
The installation instructions that start at http://ionicframework.com/docs/guide/installation.html are pretty good considering the number of technologies involved in getting up and running (npm, the Java JDK, the Android SDK, Cordova, etc.). But as those technologies changes so does the install process, even if the change is slight.
The first issue I ran into occurred when I tried to run the "ionic start" command from the command line. I got a two-part error message in the console. The latter message, cast in red text, suggested that my version of Cordova was outdated: not likely since I'd installed it 30 minutes ago. The earlier message said "could not create work tree dir" and cited a nested folder under my user account folder called "plugman". Fortunately, someone had posted the solution (the need to create the missing directory) in the Ionic forums: http://forum.ionicframework.com/t/unable-to-add-plugins-perhaps-your-version-of-cordova-is-too-old/3807/10.
The next issue came up at the conclusion of the install process when I tried to run "ionic emulate android". Again, the final error message wasn't that helpful; the earlier message stated that "abd" was not recognized as a command. The reason that happened was because the instructions say to declare the ANDROID_HOME Path variable to point to the "tools" directory of the Android ADT bundle, but Google moved the adb executable from the "tools" directory to the "platform-tools" directory (they left a note in the tools directory about the move). For Ionic to work you actually need pointers to both the "tools" and "platform-tools" directories in the path.
The last issue was particular to my machine. I had a pre-existing Android Virtual Device (AVD) configuration from an earlier install of the Android Eclipse plugin, and when Ionic tried to use that AVD it complained that a kernel file was missing. I ended up launching the AVD manager from the command line ("android avd") and using the "Repair" option to repair the AVD, and that apparently fixed the problem, and the emulator opened at its typical glacial pace.