The into fn loses metadata.
I'm seeing some team members (myself included) rely on metadata. Metadata has been incredibly useful in some cases, however the silent destruction of metadata by core clojure fns (into, walk, etc) have been a source of increasing complexity and confusion.
A team member could start relying on a 3rd party library that used 'into'. Actually, the into fn could essentially be added to the code base at any time in many ways. Wrt metadata the potential for incidental complexity increases exponentially as more developers and libraries enter the mix.
One of the reasons Clojure has worked for us so well is because it has greatly reduced incidental complexity.
IMHO the 'into metadata bug' seriously limits the usefulness of metadata and would love to see the into fn fixed in 1.4.