The problem with persona is that it sends the same identifier to all websites (nominally your email address). That makes it super-easy for those websites to feed your activity to a central tracker like DoubleClick which will consolidate all usage information from all DoubleClick affiliated websites. Persona would be a lot more privacy-preserving if it generated a unique identifier for each website. A "persona" for ea…
To be fair, this is not really Persona's responsibility. Pre-Persona, if you used the same email address on every signup, you already enabled this sort of tracking. With Persona, you'd have to take the same precaution of having multiple Personas if you wanted multiple personas :).
Also some people do want a single identity for the entire Internet, or at least a lot of it. Many people use the same real-name identifiers on HN, Github, LinkedIn, etc. professionally.