Documentation and support are the first two that come to mind – I would consider them very important if my income depended on ART.
Then library management is non-existent and metadata support could be better. You can do both with digikam, but the integration of the two is a bit rough.
It is also true that e.g. Lightroom is probably faster to use as it has a lot of “smart” controls that seem to magically do the right thing for most people most of the time…
But these are all things for which I have very little interest (except maybe for more magic – who can be against magic? 
), so it’s unlikely that things will improve anytime soon.