Beta is more finding the bugs and the flaws in a mostly completed product. A good 90% of a project is usually done before it gets released into beta. At least that is how it is with MMOs. (Closed betad Cataclysm, rift, Warhammer online, FFXIV2.0 to name a few).
The art work for some things may be different, and explanations and text is different, but generally the gameplay is 100% locked in and changes won't be made unless bugs are found.