Bread is serious business in France, but not all bread here is good. Even the simple baguette has huge variation. Now thanks to my friend Marc I know how to articulate the difference between the good and the bad.
A good baguette starts with an open crumb, an airy interior texture with different sized holes. This structure is the result of careful handling and longer proofing and is one of the visible marks of a good baguette. Of course what really matters is the taste and texture, but the easily-visible crumb structure (miette) is a good indicator.
I've been buying baguettes all over town as an experiment. And almost every shop has mediocre baguettes, including the fancy gourmet store (1.05€), the place with fabulous croissants (0.90€), and the permanent boulanger in Place Maubert (0.90€). But the best bread I've had so far comes from just around the corner on the tourist-swamped streets of Ile St. Louis (0.80€). It has enough flavour you can enjoy it plain, good chewy bite, and yes, the open crumb.
See also this sinister explanation for lousy Parisian bread