Your suggestion of "One trash can to rule them all" fits in with the way most of us are used to dealing with deleting documents on our computers - or I guess the Recycle Bin on a PC (although I hate the term Recycle Bin).

However, I think the way WordPress is currently doing it at an object level is slightly tidier and fits in with the All | Published | Drafts | Deleted way of navigating the admin. If I delete a comment by accident, I can stay within the comments section to restore it.

Notice, I used the word 'Deleted' as I prefer this to Trash. When you delete something it is usually because you DO want to delete it forever. The 'Trash' functionality is a safety net in case you realise you have made a wrong decision. Ideally you should very rarely need to go in there to restore anything.

Then if you do need to go into the 'Deleted' view the options are to 'Restore' or 'Delete Permanently'.

If you were to have an overall Trash bin section where everything goes, it would probably have to live as a new main menu on the left and I would prefer not to clutter this navigation any further. I'm sure it would be easy enough to create a plugin to collect all deleted items into a delete section if you prefer?

Oh, and I guess someone would have to design a nice Trash Bin icon if it was a new section?

:)