A lot of small things, for example editing multiple parts at the same time (select multiple parts, E, change, lets say the value of multiple resistors, enter). Or how the power ports are horrible to use (you have to create a new symbol for each new net (these should really just be some sort of global label, with a power icon). Or a better, iso standard footprint generator. Being able to have multiple sheets open at the same time. Just small things like that. As for the UI, actually, just steal the blender UI concept, that modular window concept is great, though, if you define "user friendly" as, "The user immediately knows what to do when they open the program" idk what would help with that, it seems pretty on par with other pcb design tools in that aspect.