Feel free to list functions that idi can do that EM can't.
I'll try to keep the list non technical as much as possible since the devil is often in the details. I won't mention differences in features that both offer.
I hope my knowledge of EM is still up to date so correct me where I'm wrong.
1. Versioning (managing derivative files).
This feature is unique for IDimager. With versioning you can maintain versions in a single set of images which are considered a single unique entity for the database. When you assign a keyword (catalog label in IDI terms) to one image, then that is assiginged to all images in the version set. Apart from creating version sets, you can also work with version place holders. This means that you can identify the role of each version in the version set. For instance you can define which of the versions is your print version, your web version, your mail version, etc. You are free to define as many version place holders as you like.
Besides versioning IDimager also supports Stacking. With Stacking you can group images together while each image in the stack is either a single image in the database or a specific version from a version set.
3. Hierarchical catalog labels
Speaks for itself. I believe EM2 will also offer this
4. Relational keywords/catalog labels
With relational labels you can define how your keywords/catalog labels relate to each other. For instance; if John is married to Jane, then you can define a relationship between these two labels and describe their inter-relational role; e.g. John relates to Jane as Husband to Wife.
5. Area tagging
Not only will you be able to tag your images with catalog labels, but also areas/parts of your images. It is the ideal way to define parts of the image that relates to your tags, or just to describe a part of your image that needs additional information.
6. Better XMP support
IDimager is built on XMP from the ground up. IDimager is completely XMP driven and when writing metadata, IDimager will store the XMP and also map the relevant XMP data to IPTC and/or Exif where applicable.
In addition IDimager offers features to map catalog labels (read keywords) to a predefined XMP property. This way you are basically writing part of your metadata just by assigning a catalog label to the image. For instance; you have a catalog label "San Francisco". Then you can define that this catalog label should be mapped to the photoshop:City XMP property. Then when the metadata is sync'ed to the file, the XMP properties are filled in for you. When you work with David Rieck's CVK then IDimager will automatically assign the XMP properties to these catalog labels.
Apart from these benefits, IDimager offers 100% extensible XMP. This means that you can define your own XMP schema and make them part of your Image Details. Your custom properties will be written to XMP just like any other XMP property. In fact this is what eXtensible metadata (XMP) is all about.
7. GEO Catalog Labels
In IDimager you can assign a GEO location to your catalog labels. Then as that catalog label is assigned and the metadata is synced, IDimager will automatically GEO tag your images. For instance; you have a catalog label "Statue of Liberty". Since this is a fixed location you can assign a GEO location to it. Assign the catalog label and sync and your image is GEO tagged.
8. Open architecture
Nothing (and I mean nothing) is proprietary. you have full access to any information in your catalog. Either through direct database access, export features to CSV/XML, or through scripting.
9. Integration with Photo Sharing services
IDimager includes upload features for the most popular photo sharing services like FlickR, SmugMug, and ZenFolio. The upload features will also set your descriptions and keywords on the service, even if you decide not to include metadata in your images.
Apart from designing a catalog label hierarchy you can also create so called "portfolios". Portfolios contains hierarchical collections of images and are generally used to create sets of images for a specific purpose. For instance, create a portfolio for the images on the website, or portfolios that contain the customer selections for each project, etc etc
a. Database size can grow up to 2 Terrabyte
b. Integrated Downloader; including features for mirroring, catalog label assignments on ingestion, initial metadata descriptions, DNG conversion, matching RAW+JPG combinations in version sets
c. 100% Unicode support
d. Better support for files on network drives (EM 2 will improve this I believe)
e. Filter scripts, Sort scripts, search scripts (if a sort feature is not available, it can be scripted; the same goes for Filtering and Searching)
f. Synonyms (I'm not sure if EM supports synonyms or not)
g. Support for Lightroom hierarchical keywords
h. Stored groups; these are stored search queries that are accessible in the Catalog Explorer as would it be a single catalog label.
I think I highlighted the most important functional differences (though I'm sure I forgot a few). Again, please feel free to correct me where I'm wrong.
(this list is based on IDimager Pro features)