Just one complaint. The visual appearance could be improved. I mean, it is really hard to follow the cloud of select elements. Especially when SEARCH and BATCH is open at the same time. I think it needs a better differentiation, maybe different colors for the two main buttons and select-groups. The collapsing areas seem to melt, it is hard to see the relation.

We've already a ticket to improve this. We'll be encasing each in fieldsets or adding a separating border of some kind.

BTW, do you have to follow J! UI guidelines and capabilities (Bootstrap v2) ? Or are you free in appearance and technical stuff (BS3 + own stuff like custom select boxes and more icons)

We're using Bootstrap 3 and all styling is independent from Joomla. For our icons we're using fontawesome so you've the entire fontawesome library of icons available to you.