A couple for me.
Firstly, building efficiently. No problem going between the planets, but my rockets are always brute force and ignorance
Secondly, transporting rovers effectively and modular surface building. I can never picture it when I am designing in the VAB. For rovers (and other cargos) I can never picture how to properly secure the payload without it looking daft, or wobbling, etc. With the modular bases I can land in a decent target area, but never figure out how to move part a over to part b to dock (join) them into one unit. Instead I have lots of single launch buildings all close by, but never joined together! Also, my rovers, when I do manage to get them carried, look like a mess of girders
I don't have an 'artistic' mindset at all, so visualising my builds is frustrating