It's a combination of not having injuries (although now that changed with Hughes getting hurt), player improvement (Hughes, Arenas and Jamison are all having either career years or close to it), and the pieces fitting better (jamison fits in much better with the lineup than stack).
just like it wasn't all (or even mostly) his fault last year, it's not all his doing on why they're better this year.
I think he's an incredible coach though. I wish the Sixers had gotten him last year (it was between the Sixers and his hometown Wizards). But the turnaround is many factors.