Hmm ... I also crashed after upgrading today (registered with new key, etc.)
The crash has not repeated after re-opening the subject file (made with TB 5.12.2 I think.) After some exploration, I was unable to select and move individual notes reliably. I also noticed that my trackpad cursor displayed a hand (for dragging) almost exclusively. I have not been able to select a group of notes graphically by selecting an area that intersects the desired notes ... is that a new thing? (I can, however, Command-click or Shift-click for range selection on multiple notes, just no "rectangular area" selection.)
When I restarted the OS, the ability to actually select a note and then move it returned (but still no rectangular area selection on the map at all) ... so not sure what's happening.
The new design looks really great, though, and despite these oddities I like what I see (overall organization, menus, appearance, etc.)
I'm on Mavericks OS X 10.9.3, Macbook Pro i7, 16 GB RAM. I'll keep exploring and submit a crash report to Eastgate if it recurs in the meantime.
** Update: Now holding the Option button down allows area selection, so unsure of what the deal was there. I switched to TB 5.12.2 and selection worked fine, when I returned to TB 6, voila ... working! Maybe a subtle file load/save issue? Anyway, seems to be working now

*** Update again: the "Keyboard Shortcuts" post identifies holding the Option key down in Map view as allowing area selection, which makes me think I'm just reacting to feature changes, perhaps. I guess all good things require some patience?