I just found another page I can use as sort of a control query against "widget tutorials." Let's call the query for this page "green dog tutorials". This page used to be #2-3, but now it's in the 600s for both "green dog tutorials" and "green dog." Add the words from my non-EMD domain name in there, and it rises to the top.
It would appear that it's my "tutorials" pages which are suffering most. But why? It's not about quality of content - if it was, I'd have been hit by Panda. I got hit by Penguin, which means Google thinks I'm spamming. Do they think I'm over-optimized for "tutorials?" If so, how? I have extremely few inbounds that use that word in the anchor text, so it has to be on-page... but I don't use that word much (I have pages where I use it more that are still ranking well). Besides, as was my whole point with this thread, I think Penguin has to be looking at something more subtle than keyword stuffing, since the old algo could deal with that.
I just don't know. I do wonder if it's the whole "tutorials" niche being rearranged because of some broad pattern amongst hundreds of sites - in which case I'll never figure out what it is because I don't have access to all the data Google has.