It's definitely a pain in the ass - even moreso that the forum posts have an auto-save feature that for some reason didn't make its way into the blogging module of the software as well. A couple of times I've hit Backspace while the text window was out of focus (from uploading an image or doing something else outside it) and that cause the browser Back History function to trigger and bye-bye 30-90 minutes of work, at least.
However your issue sounds like a bug. So you had a large entry, went to save as draft and only part of the entry was actually saved? Can you reproduce this? Copy/past a large chunk of text and use the Save Draft to erase some of it? I just pasted nearly a thousand lines of text into an entry and it was all there when I saved it as a draft.
But yea, I tend to nowadays write everything up in a text editor then paste it over into the journal entry box. Just in case.