Plugins, especially EQ
As a pro user I have used all the major DAW programs. I am currently working with SADiE (piece of crap) and in my previous job Logic 7 (fine piece of engineering).
My suggestion would be to rewrite or create new LADSPA plugins to give Ardour the power to be a truly great piece of software.
The first would have to be an eq at the top of the channel strip like Logic, the gui they use is incredibly powerful and intuitive, a track can be tweaked in seconds and the preset can be DnD'd to other channel strips.
The other plugins that I have used have also lacked clear GUI's and advanced features, it would be great to have a simple powerful set of tools like Logic that fulfill all the core requirements.