Lack of interaction on Assassins

*disclaimer, this idea was spurred on by Zed specifically, but applies to the whole "class" *

So let me set a stage for you. An ADC ( and a fed jungler ( ) go to top lane to push the lane to the tower. The enemy mid lane is 0/9 after a particularly rough laning phase. The ADC went slightly better than even and the jungler got lucky on all of her early ganks and is doing quite well. The enemy mid lane has about 1.5 seconds in between cc from Vi's ult and q to use his abilities. The MF still dies.

My question is this: is this how the game is supposed to work?

Where do we draw the line on assassin interactivity. If they are too interactive they are not as effective as fighters and bruisers who can take damage before dying. Too little and they are just point-and-click murder-bots who aren't any fun to play against.

I recognize that finding a way to walk this tightrope while also creating a compelling and unique character must be incredibly difficult. I also think that Riot needs to be clear where they stand on this question.

So let's talk about it. Which champions (or specific abilities) do you think exemplify a "well-designed" assassin? Where should the balance of interaction fall? How can Riot make assassins that feel rewarding both to play and to play against?