You missed that Digikam can actually integrate it's metadata with the desktop: there is the option to store the meta-data in Nepomuk and keep it synchronized.
Also missed the Light Table, extensive color management, gphoto-based integrated Camara support, the fact that there is no Free application which is so powerful when it comes to handling RAW files, duplicate search, the super-cool fuzzy search (quickly drop an image or even draw a form and Digikam finds images which look like what you've just drawn), stunningly fast performance on even large databases...
And yes, everyone hates the vertical tabs. I'd be happy if they replaced them with a horizontal tab bar on top, but it'd make the minimum width quite big I suppose.
Frankly, your review, while barely scratching the surface of Digikam, clearly shows the reason I don't use digikam much. Which is ease of use. I'm incredibly lazy when it comes to pictures, I trow 'em in a folder and that's it. Digikam is awesome if you want to put in some work but fails when you struggle with things like you did, the face tagging for example. It could use a new interface.
Maybe: Only show the Timeline over the full width on the bottom and a small drop-down with the other search options ppl use less frequent. On the right, reduce the number of 'tabs' so you can put them on top. For example, integrate properties with location, exif/etc and tags in one tab, histogram and ICC in a second tab. Should work better, I think.
Anyway, what do I know of user interface design, I'll stop ranting ;-)