From what I understand, Alpha is when only some of the features are implemented but Beta is the stage where the game would be "feature complete". However, with lots of bugs and things wrong that need to be fixed and improved. Beta is still low quality and needs to have a lot of problems fixed before it is at a "releasable" level of quality.
If you're playing a beta, then in a way you're serving as a tester and the aim is not to enjoy it fully but to find problems, and provide feedback so the problems can be fixed by the developers.
I think you should only participate in a beta test program if you are doing it in that spirit. If you want full enjoyment, better to wait for release. Personally I won't play the beta, I want to wait for the final product and just play that.