I guess depending on what you mean, this is partially do-able in Mail.app, as is.

What’s nice (and in my opinion much improved) about Thunderbird flags is that they are a) numerous (Mail/IMAP has one digital flag: a message is either flagged or it’s not flagged), b) semantic (they mean something—don’t just change the background color, etc.), c) keyboard-assignable (remember, we have multiples here, not just one CMD-SHIFT-L), d) customizable.

So theoretically (and I haven’t really thought about this that much), I could have an “URGENT Work-related” email and an “Answer Anytime” email live in the same “Email to respond to” mailbox. This keeps everything under my nose (not buried in a folder someplace); they’re functionally the same (they both require an email response) but their semantic flag status lets me filter to just what I need without changing views (or waiting for Smart Folders to churn).

Being able to blast through a screen of messages with just keyboard commands every hour or so is basically my Email Valhalla.

(I’ve actually developed something similar to this by combining Mail Tags and Mail Act-On that I’ll be writing up soon.)