The reason people hate Zed is the same people hate Nidalee

He's a generalist. He's supposed to be an assassin, but he's so much more. That last CLG/TSM game exemplifies that he's strong at a lot of different things.

• He has a safe laning phase, so if he doesn't get a kill in lane or from roaming he will not be starved for gold.

• He's a strong lane bully, so he'll likely force you out of lane with relatively little risk.

• He's a strong, safe split pusher.

• His core itemization allows him to be effective against both tanks and carries.

• As an AD champion, he can miss a large part of his combo and still deal relevant damage with AAs.

• He's not punished for failing to assassinate a target. He can still contribute to teamfights through poke and his basic abilities.

From solely an assassin perspective Zed is well balanced IMO. It's just that he does so much else that it doesn't matter. I saw a lot of people complain when Riot nerfed his AS. Honestly I think that was the right approach (nerfing his auxiliary roles), but it didn't go far enough. After being nerfed his base AS is average, and his AS at 18 is in the top third. Nothing has been done to address his ability to CS laughably easily.

That's not all just Zed having his kit. It's Zed's items as well. BotRK was designed for MADCs to stay in range and to counter the warmogs meta. It has since been used for extra burst in duels and for extra split pushing power. Armor pen, with any common sense, should be a strong counter to champions that build armor. Yet armor pen is still strong regardless of whether or not your opponents build armor. Now look at assassin itemization. All core AD assassin items have armor pen on them.

Adding to the itemization issue, all of Zed's core items have effective build paths. Longsword is 360g. There are 5 longswords between LW, Youmuu's, and BotRK. The largest gold gap for all of those items is LW's combine cost at 1065g, which is usually the last item built. The largest gold cost for all of Youmuu's is Brutalizer's combine cost at 617g. The largest gold cost for BotRK is 900g at the final combine cost. While finishing those items you're sitting on Cutlass and Brutalizer, both of which are strong items in their own right.

TL;DR: Zed is strong because he's good at a lot of different things. That's both from his kit and from poor item design.

Edit: Let me clarify that

1. Being a generalist does not mean you can do everything. It just means you can do a multiple things, and more so than the norm.

2. Zed the champion alone is not a generalist. Zed with armor pen and BotRK is a generalist. They enable him to go beyond his assassin role without sacrificing anything.