I like the idea of tabs but for a slightly different reason, which I'll get to.
Before that however, it seems you have two separate feature requests here:
- Tab bar (instead of left list)
- Advanced labels
For the "Tab bar":
I still haven't gotten into deluge's source so I don't know... but I would hope a "rewrite" of the gui layout wouldn't be to difficult. I imagine the difficulty would be in adding facilities to switch between layouts. Another improvement in this area couple be to enable split screens to show multiple torrent lists next to each other.
For the "Advanced labels", I'm sure everyone's had ideas how to fix (make labels non-lower case and accept spaces

) and improve the label plugin, but I agree more flexible labeling is needed to make Deluge the full featured client it's trying to be.
But back to my reason for going to tabs. Coming from Azureus's tab based interface to deluge's utorrent like interface was just annoying. Azureus's main tab shows all torrents, split into two "well defined" torrent states and splits the lists accordingly. By having a different list of downloading and seeding torrents lets you have different column setups for each. A primary advantage for instance is hiding the %completion bar or download seed for seeding torrents, leaving more room for the name and other status columns. The tabs are then dedicated to individual torrents (or option & plugin pages) and details are shown on those pages, with an extra row of tabs, instead of a shared "options" pane whose contents are swapped depending on what's selected and always taking up screen real estate always forcing you to choose between making it small to just show the few lines of options you normally use and expanding it to see a full list of seeders for instance. (Maybe this is one of the things giving deluge/utorrent a memory advantage over Azureus, besides Java).
I realize that this could be a significant change to the interface design which will also tie to a lot of things below the surface but I just wanted to get this point made and out there.