Safari's Tab Overview isn't bad, esp with the pinch gesture, but Orion's clear interest in vertical tabs prompts me to recall how much I really appreciated OmniWeb's Tab Drawer with vertical tab thumbnails.
The contemporary approach would be a Sidebar not Drawer of course (e.g. picture Preview app with page thumbnails shown.)
IMHO this would be a great option to have back in a WebKit browser, but of course understand the need for focus if this might be a less commonly shared desire.
A compromise approach might be the addition of Tab hover thumbnails (like Safari) that were then avaialable on either horizontal AND vertical tab bars.
Thanks for all the effort going into Orion — it's looking so promising!