If I understand correctly, SE reproduces its user-produced content using a CC-BY-SA license with additional restrictions: namely, from point 3 of the TOS,
In the event that You post or otherwise use Subscriber Content outside of the Network or Services, with the exception of content entirely created by You, You agree that You will follow the attribution rules of the Creative Commons Attribution Share Alike license as follows:
a. You will ensure that any such use of Subscriber Content visually displays or otherwise indicates the source of the Subscriber Content as coming from the Stack Exchange Network. This requirement is satisfied with a discreet text blurb, or some other unobtrusive but clear visual indication.
b. You will ensure that any such Internet use of Subscriber Content includes a hyperlink directly to the original question on the source site on the Network (e.g., https://stackoverflow.com/questions/12345)
c. You will ensure that any such use of Subscriber Content visually display or otherwise clearly indicate the author names for every question and answer so used.
Although the wording "You will follow the attribution rules [...] as follows" is a bit misleading, these are indeed additional restrictions; for instance, CC-BY-SA says nothing about
nofollow, and asks for hyperlinks only when reasonably practical (at least in the newer versions; the older ones did not specify anything. The TOS never specify a CC-BY-SA version number, and it contains links to both the 2.5 and 3.0 version). [EDIT: apparently now they have updated everything to 4.0].
Adding additional requirements is apparently frowned upon by the CC folks, and they require that you stop using their name and logo in that case:
Can I change the license terms or conditions?
Yes—but if you change the terms and conditions of any Creative Commons license, you must no longer call, label, or describe the license as a “Creative Commons” or “CC” license, nor can you use the Creative Commons logos, buttons, or other trademarks in connection with the modified license or your materials.
My understanding is that Stack Exchange sites should stop using the CC name and logo outright. Is this correct?