I am going to be getting a digital camera within the next two weeks (hooray!) and while I might be wrong, I don't think that MC does any interfacing with digital cameras (?). It should have TWAIN interface and download from cameras as a minimum set of features to make it a worthy image library.
Second: Media specific fields. MC is very obviously audio biassed and quite frankly, not many of my photographs and pictures have a BPM, a replay gain, a composer, a bitrate or even a duration that I particularly care about. The ability to hide fields that don't make sense so we can see the wood for the trees would be a good second step.
Third: Features like creating thumbnail webpages, index cards, format conversions (batch, of course!), some basic image tools (cropping, rotating (not just lossless jpeg), resizing) and some basic effects (perhaps build this on a plug-in architecture so that more effects etc. can be added) such as b/w, sepia, negative (? these are all found on some digital cameras which is why I mention them), sharpening, etc.
There is more, but those are probably the main differences between MC and something like ACDSee.
P.