Tooltips would be nice (and make sense). The help is however to big and complex already. I have suggested to have simple and expert modes to reduce interface clutter and learning time: Simple vs Expert mode.

I just posted a solution to various icon and usability issues. See: Update iconography and UI.

I also had this problem on my first run with TO. When and if possible, showing a group being dragged via title and total tab count ("Window - test stuff (23)") would be much better.

Perhaps it's better to use an 'on-mouse-drag-start' event to replace the visual somehow?

Would it be worthwhile making it more noticeable? Most 'web apps' tend to have a fixed bottom bar that is consistently easy to find. I don't think transparency enhances the experience at all. Also, transparency fading obviously is not as lean (CPU friendly) as something without animation. You could also re-consider placement. Dragging up is harder than dragging down or right. An opaque fixed left vertical bar might make more sense and take up less space!