Genjutsu is measured by Intelligence.
Everything in a genjutsu need to be shaped and manipulated to a point it seem realistic, try imagining yourself kicking a man off a cliff, every detail such as grass, insects, water. Tough aint it?
That's why I think the more creative and intelligent you are the more powerful your genjutsu.
Like Kurenai example, she uses genjutsu to confuse and scare her enemies even though she don't have Sharingan.
Itachi is a good example too, his caliber with the Sharingan is so mastered he can make an 3-Tomoe Sharingan Genjutsu as strong as Tsukuyomi.