sudo poweroff in terminal, got locked out of SSH, and still had to power off from the web UI. After I powered off from the web UI and took a snapshot, my droplet powered on as soon as the snapshot finished (without me clicking "power on") and my SSH access was reestablished.
I don't think the problem is completely fixed. I had to request console access from the Web UI for it to recognize that I powered off from terminal.
The "poweroff" command worked perfectly for me when I used it after logging in as ROOT (so no need for sudo). When I went back to web interface, the droplet had indeed been powered down.