Ignoring the colour (red on white VS white on blue) and the distressing of the Debian logo vs the cleanness of the designmodo logo there's the opening of the spiral - Debian curve starts at 6 o clock, while designmodo starts at 4:30 or 5 o clock. And the inside of the Debian loop goes further.
EDIT: https://secure.gravatar.com/avatar/7d9027189b18855f5f2ddeb7d...