Gary Bernhardt says that Bazaar is different Mercurial, because it stores more metadata about the tree rather than just the state of the tree. This keeps you locked in a certain version of the repository, keeping them from being able to add metadata. Mercurial just stores the state of the tree, so you get a better chance of keeping up with the versions.
Interesting! I'll have to keep this in mind if Mercurial is as easy to use as Bazaar.
[Update: Gary has put a comment on this entry that gives more detail about what he means. Well worth reading it.]