Missing Metadata Navigation panel

Had a bit of a brief stumper just now.

I've started using the Metadata Navigation panel more in some of my libraries. The tree view is somewhat useful for folder-level filtering, but the real juicy bits come in the Key Filters list.

Well, this is all well and good, but what if you configure Metadata Navigation, and then the panel doesn't even show up? A quick survey of the Interwebs reveals that everyone thinks that there are only two causes to this issue:

1. You turned off the feature
2. Your masterpage is borking it

Two very reasonable conclusions, but in my case, I had two libraries in the same site and one library had the navigation and the other didn't! Egad!

After doing some comparisons (and also noticing some odd webpart behavior from a week ago when I first set these libraries up), I realized that the Metadata Navigation panel appears to only show up for List webparts in something resembling their default configuration (more or less).

For the uninitiated, view pages are simply web part pages that have data view parts with the view information preconfigured based on the ViewFields collection. To a limited extent, you can modify these pages and add extra webparts and such. Apparently, the webpart in one library had its toolbar set to Summary, and another set to Full. I updated the Summary toolbar part to Full, and the nav panel reappeared. I turned it back off, and the panel stayed, so it's not a dependency on the toolbar setting at the very least, but there may have been another missing property in the web part (this library and its singular view had been untouched since our migration to 2010), and it appears that changing the toolbar mode reset that.

I hope this saves someone some agony, but then considering how narrow my particular issues tend to be, I have to laugh at myself about it all...