Observations based on Think Secret’s preview:
The thumbnail and list view name labels are not very useful for sites with a standard prefix; Think Secret’s thumbnail and list view screenshots illustrate the problem. Because so many sites use layout templates for content, the thumbnails are not distinctive enough to differentiate several pages from one site. It is in these cases that the titles become important. Safari rather smartly removes common prefixes when multiple tabs are open, making them much easier to identify. This proves very useful on the MacNN forums, I’ve noticed.
Like previous versions, the main toolbar uses color and shape effectively as means of differentiating icons. Safari and Firebird absolutely suck in this regard.
Per site text size manipulation will be great for those sites that use tiny fixed-size fonts.
Are the bookmark Collections spring-loaded? If not, why not?! Ideally, these would work similarly to the 10.3 Finder’s spring-loaded sidebar, with the hovered folder opening in the same window.
Do tabs open to bookmarked pages use the bookmark’s title? How about Page Marked pages?
All in all, OmniWeb 5 looks like it will be the browser for those who want powerful features without giving up a native OS X interface.