Is there a way to browse by keyword, i.e., to pick a keyword used in the database and then see all documents that have that keyword as part of their metadata? Or at least to find out what all keywords exist in the database? Could this type of functionality be added if not? Thanks.
With ‘keyword’ you mean basically a ‘tag’? If yes: Wait for one of the next releases. If you simply mean a word present in the content of a document you could create a smart group to accomplish this.
Or use the Tools > Search… (Shift-Command-F) window. 
Sorry, I didn’t realize there were so many ways to interpret my post. 
I literally mean “keyword”, as in the metadata field of the same name.
Browsing is also important. The main point of my request is to find out about keywords I don’t currently know about. So unfortunately, Find will not help me with this. Currently, the only way I am aware to do this is to write a script to go through every document in the database and generate a list.
Beyond some sort of concordance feature for keywords, it would cool to easily analyze which keywords are most/least popular and given a keyword, what other keywords appear in the document set.
Hope that clears up my request. Thanks.
Still not certain what you mean since DEVONthink doesn’t support its own “keyword” metadata type. Maybe you mean the “Keywords” attribute that can be searched with Spotlight, etc?
If you’re seeing this “keyword” somewhere in DEVONthink maybe uploading a example screen image attachment should make it unambiguously obvious what you mean.
I don’t know about its own “keyword” metadata type, but it does have a “keyword” metadata type. 
Notice that this is for a web archive. PDFs and, I believe, RTFs also have it.
Separately, I really hope this doesn’t mean I am the only person using this feature. I really like it and would hate to see it wither due to lack of interest.
That’s Spotlight’s keyword metadata, and you can search for all documents with specific metadata by copying or entering the keyword in the Search window (as sjk mentioned) and click on the ‘metadata’ button. I’m not aware of any way to search for a list of all keywords in the database.
Spotlight indexes Keywords metatdata but DT could be reading it directly from files; it existed before Spotlight. It also shows up under the More Info section of Finder Info windows. Seems popular in PDF files. In .webarchive files it looks like:
I wish that “Meta Data” radio button was labeled Metadata, and all other two-word instances of it were changed to a single word.
Me neither; feature request? Doing some quick test searches for metadata Keywords with my largest database returns all results too quickly for them not to be indexed. But they’re not displayed in the DT Keywords list:
That its tooltip label is Keywords added to my uncertainty about which ones psemlohq meant, before seeing the helpful (thanks) Info and Properties window image examples. I was only thinking of the Spotlight-indexed keywords as “metadata Keywords”, not the “DT Keywords”. Discussion would be easier if we can agree on some non-ambiguous terminology for differentiating them.
Btw, I recently noticed the Meta Data page of Help has “an (incomplete) list of theoretically supported fields”.