Thank you for the responses. It tends to confirm what I have been feeling about Zenfolio.
On a practical note, I noticed that Jeffrey Friedl offers a LR plugin ("Export to Zenfolio"). Does anyone know if this plugin facilitates uploading images to Zenfolio beyond using the Upload button in Lightroom 5's Web Module?
Nobody answered you question here and I don't know if you found out by yourself. So if not.....
The plugin you mention is a publish service which has nothing to do with the web module. The plugin will logon to your Zenfolio account and you can make folders on the site and upload directly from the plugin.
Also what is great you can see directly in the folders inside the plugin folders which photos you have in the folders on the Zenfolio site. See the screen shot where you can see an example of one of my Zenfolio publish services where you can see pictures that has been reedited will be marked to be published again in which case the photo on Zenfolio will be replaced by the new version. You just drag the pictures you want in the different folders on Zenfolio into these folders insode of Lightroom and click publish and voila the pictures are automatically uploaded to Zenfolio.
In the plugin you specify the resolution and sharpening of the pictures published via that plugin service (as well as color space, quality, etc.). So if you need a different resolution for another folder, you need to create another publish service using the same plugin and simply login to Zenfolio and specify the different resolution. You may need for some folders full resolution and for others a low rez picture. You can right click on an image inside the folder and show the picture on Zenfolio. This will luanch your browser and show the picture on the website.
Pretty cool and I use such plugin publish service for Zenfolio and for my main website hosted by Smugmug.