I'll try to clarify a few terms:
An Alpha code may have incomplete features, concept testing, stability issues and so on. During the Alpha stage, features may be added, removed, changed etc.
As we mentioned in our announcement, the current testing is concentrated around the new, high resolution Earth, testing performance, stability, compatibly and of course, bugs
Betas have a more complete features set, usually have less obvious bugs and as Beta testing moves on, more stressful and exhaustive testing and bug-fixing cycles take place, until we reach the first Release Candidate which is mainly bug-free, but still being tested until we are satisfied and move to a Release version.
As everybody knows, Release versions are usually still buggy but the bugs are much harder to find, much less critical, and more obscure, basically because the Beta team cannot possibly emulate the real-life situation where hundreds of thousands of users play the game under thousands of different conditions. That's when the Service Packs come to be