Why People Hate Lee/Fizz/Zed/LB/etc.
Every couple of days, depending on the nerf cycle, there is a new thread about the need to rework one of these champions. Regardless of their relative strengths and weaknesses in their roles, I think it can all be boiled down to a single trait:
In the hands of a semi-skilled player, they are ridiculously safe. They will outrun you in every chase, they will dive bomb and leave before your skillshot CC has time to hit them, they can move around the map like its an open field, and generally your only option is to kill them before they kill you (hyperbole, but still kinda true).
I'm not saying they need ALL of their mobility removed, but Riot might want to think about whether it is a good idea to have any champion be that safe (again, in the hands of someone with a reasonable skill level). Tanks can be chipped away from a distance, ADC's generally have much more limited mobility and are squishier, and mages are usually skillshot reliant and immobile.
EDIT: As many people have pointed out, assassins are designed to jump in and burst squishy targets, and almost all of them have the tools to do so. My complaint is about safety, and the ability to jump in AND out combat without an ult or kill reset, but still doing considerable damage.

?
stealth.

R 3 charges.

gap maker/closer reset on kills.
are the only 2 assassins sitting duck after the attempt. I'd gladly trade the short CD for a reset on kill, but that'd only bring more QQ.