Bugs....absolutely. Stability is the issue here. Betas are for working out quirky problems that cannot be duplicated in a lab. When your vendors and test audience don't have a stable beta model to work with, it pretty much renders the entire process moot. This is a big problem if MS tries to go to market with an unstable Vista. It would be Windows ME to the tenth power.
Yes, I understand the difference between beta and final versions. Beta is where the bugs are supposed to be worked out, not final versions.