Here's the Reader's Digest condensed version...
A good brevet bike is one you use on a brevet with little or no hassle or pain.
This means the bike is:
- reliable and comfortable
- has enough storage capacity that you can carry what you need for the distance and conditions (including clothes and tools) and/or store what you no longer need.
- and since the longer brevets require lights and usually involve sustained night riding, a bike used for longer brevets should have reliable long-lasting lights.