I think it would be interesting depending on how fast mods are hemorrhaged. If the majority of the large subreddit moderators left overnight it would be very difficult for their shoes to be filled. It takes a different skillset than moderating smaller subreddits. The transition period where the new mods learned how to deal with it would be rough.
If the bottom fell out overnight it would also spark outrage amongst the users. If it was slower the userbase probably wouldn't notice.
I actually love stumbleupon! It's what brought me to reddit in the first place. I would usually find new subreddits by browsing the comments section. A lot of people would link to some relevant subreddit. Another great way is to follow any links in the side bar of a subreddit (or to browse related tags in the case of hubski).
If you want to contribute new content I would suggest following more blogs and news websites. Blogs especially seem to be content creators.
I heard about a bunch of reddit alternatives including this one. I'm not making a break from reddit, but I've always enjoyed discussion based threads. This place seems like a cool place to try. It reminds me a lot of r/truereddit.