A few suggestions to make image metadata more usable:
1) Using Metadata in Library functions
Current situation: The LR filter, smart collection, and file renaming mechanisms all allow access to subsets of EXIF/IPTC metadata - but the subsets are incomplete & different (examples - you can use "Headline" in a file name but not as filter or smart collection parameter and you can't use "Event" in any of them).
It would be helpful to either allow access from each of these functions to any EXIF or IPTC field, or, if that is too cumbersome (too many fields), then allow the user to define a preference for the subset of fields that would be available to the functions.
For example, if you use Caption, Title, Headline, and Event, then you should be able to access each of these fields in the Filter, Smart Collection criteria, and file renaming.
2) User defined MetaData Field Lists (customizing which MetaData fields show up in the Metadata Panel) - I've found web articles showing how to create these lists - it would be a minor change for Adobe to officially document and support this capability.
3) Add a command to bulk edit metadata for selected pictures - most importantly the ability to copy or move data from one field to another (I'm probably not the only person who has used fields inconsistently from year to year). Even better would be a bulk edit function to fill or modify field contents based on other metadata for the image (something like the custom filename dialog in export).
I'm starting to look at John Beardsworth's "Search Replace Transfer" plugin which appears to solve items 1 & 3 - but these are important capabilities which should be built into Lightroom and supported by Adobe
4) This might be a stretch, but the same sort of metadata flexibility in text fields in all of the output modules. Allow the use of any metadata field (as in #1) or constructed fields (like the file name dialog) to fill text boxes in Book, Slideshow, Web, and Print. Even better if we could define multiple text boxes and multi-line text to better label pictures in Slideshow, Web, and Print (for contact sheets).
As more users' catalogs grow into the 10K's of images and more, the need to manage and clean up metadata will only become more important.