One important difference: you don't have to install Ansible on the nodes you're managing, just Python >= 2.4 with a JSON module installed (default for 2.6 and later, available through simplejson for previous versions).

Also, Ansible does not require you to mess around with dependency lists to ensure that packages/files are installed in the right order - the order is built into the yaml config file. You don't lose any capability, you just gain (and this is my personal opinion) ease of understanding the order that operations will occur in.