1* -> 2* : Easy.
2* -> 3* : Looked quite hard for me at start, went through by playing 2* maps in HRDT.
3* -> 4* : First real wall I hit in osu! actually.
4* -> 5* : Well, things start to be kinda serious.
Comin' to 6 stars, I can't say, seen I don't pass.
But if I have to resume :
2* : Density object ~1.
3* : "Oh baby it's triple" + Density object > 1 = Maps which start to really follow the music.
4* : Jump, jump, streaaaaaaaaam, jump, jump, streaaaaaaaam. Jump, jump, jump, streaaaaaam.
5* : Jump faster, stream faster, get some finger control.
You can still clear 4* even if you're not good at anything. I would say you can still clear it even if you're bad at something (in my case : stamina).
For 5* you don't have to be excellent at anything, but you really need huge basics everywhere : Aim, Stamina, fingers control, reading abilities. Missing one badly results in a map failure.
So my answer will be 4* -> 5*, 'cause it seems to be the biggest gap to me.