He's actually not that overloaded, he really isn't.
Zed's passive: Execution on hit. That's not that bad, honestly.
Q: Line-based piercing skillshot with reduced damage for each pierced enemy. Again, not bad.
W: A second Zed, duplicating his abilities from one location that he can warp to instantly if needs be. Still not bad so far.
E: AoE damage in a small radius; slows if the shadow clone hits an enemy (his sole CC). Still not seeing it.
R: Marks the target, and places a ticking bomb spell. Places a shadow clone at his current location and Zed himself moves, with invincibility, toward his intended victim. Must wait 1 second to warp to ult shadow. That so far seems the worst, but all things said: he's still not that bad.
Zed isn't really overloaded. He -seems- overloaded because his (honestly) fairly straightforward kit allows him to do more than one might think he should be able to. However, his straightforward kit means he's not too adept at doing anything he isn't explicitly built for (damage). He isn't durable. His damage isn't really that insane compared to most assassins. He's slippery, but no more so than other assassins. His main 'thing' is that he has all of this along with Energy, which helps him have a strong laning phase along with his passive.
I hate him, but he's not really overloaded as one would think.