OK, this is an important point. "Open" isn't a meaningless marketing term, it's a very specific attribute of a license. Here's the Wikipedia definition of open content:
http://en.wikipedia.org/wiki/Open_license.
For those who fear knowledge, I'll sum up here. Open content is:
- royalty free (you don't have to pay anyone to use it)
- share alike
- may or may not allow commercial redistribution (your choice).
The important part here that's probably unclear is "share alike." This means that the license has "copyleft" provisions, also described as viral openness. The Creative Commons wording is "If you alter, transform, or build upon this work, you may distribute the resulting work only under a license identical to this one." Basically you can use the open licensed stuff and your derivative work needs to be open too, and that openness can't be removed (except by self-breach).
Google any of the keywords in that spiel for more information (copyleft, Creative Commons, etc.)
The OGL was an open license. Wizards of the Coast can't revoke the OGL - everything that was declared open shall be open until the end of days. They can do things like "poison pill" the GSL to try to disincent people from using it, like computer companies try to do (Intel to Dell: don't use AMD). But they can't make it un-open. This is what makes it different from a "normal"license like the GSL, that they can revoke or change at any time.