Ironically enough, given the complaints that have sometimes been levelled at OmniWeb's tabs, there are more than a few Opera users looking for their new tabs on the side, a la OmniWeb.
OW's tabs aren't perfect, but to my mind, on a widescreen vertical tabs simply make the most sense.
While there's an aesthetic, and familiar, appeal to horizontal trip of thumbnails, this is most suited to horizontally oriented content, like video and images (i.e. iMovie, iPhoto), whereas websites do still tend to be longer than they are wide.
As at least one of Opera user in the thread above would like to see the Tab preview pane to be shown on mouse-over, akin to iPhoto's Full Screen mode, where the Photo Album contents is shown in a 'filmstrip' along the top of the screen, when you move the mouse to the top of the screen.
This would balance the space-saving of traditional text-only tabs, with the visual recognition of tab previews. While I quite like OW's Tab Drawer, if the Drawer were to hypothetically become an integrated sidebar, a la iTunes, there might be the possibility of a similar collapsed/expanded mouse-over toggle, as an option to maximise space for the web page.
Finally, FWIW I'd really like to see a Full-Screen mode in OW, very much like the iPhoto Full-Screen mode above, though with the 'pop-out' tabs appearing on the left or right.
BTW - Shirra's method of showing a loading progress-bar in the tab-preview is a nicely visible loading display, esp. in Expose mode, that would be a nice addition to OW (with some refinement.)