I think that one's on you. Just like saving a file on a computer after doing a lot of work, doing the equivalent on a tablet is a good idea. The automatic memory management may have shut down the app, or the unit inadvertently rebooted for a variety of reasons. Saving and backing up are basic common sense protections against this sort of thing. That issue is not unique to B&N's NOOK.
Bob George has a point, but some apps and applications save drafts of work-in-progress. How nice would it be if BN's note-taking function worked that way, too?
True, but even those are a usually works. Anything worth 30 minutes of effort is worth a second to save.
In order for the device to go to sleep, you have to either select for it to do so, or not use it for a period of time. So it sounds like either
(a) your device is going to sleep while you're actively using it - this is a device problem
(b) you are choosing to put the device to sleep without saving, or leaving the device sitting without saving - these are on you
Check settings to see how long your device should be idle before it sleeps. Maybe this setting needs some adjustment based on how you use the device. If it's not waiting the requisite amount of time, it's time to contact CS. If it's working properly in that regard, then you'll need to change your behavior to resolve the issue. Either way, saving out regularly is ALWAYS a good idea.
Also, FWIW, you could get an app, such as Simplenote, that automatically saves your notes at regular intervals so you don't have to worry about saving out.