For me the most important thing would be to improve the performance of d.tube and fix some bugs... Today I just logged in after a few months. Not only does it take quite long after any page is shown, but also i had to do the whole login process for 3 or four times, because somehow it logged me off after I clicked on a single video...

I guess d.tube just feels way too "clunky". It is not mature enough to be a good competitor for YT (you could argue if d.tube really needs to be a competitor, but anyways)...