Maybe you should play it before you comment on here, it is undoubtedly worse. If you had played it you would realize that even if you get the experience later on, due to the out of combat requirement, their exp requirement, and the channeling, you cannot just go about using all the experience as you please. It gets built up and you eventually end up with a ton of experience, which you cannot use sitting in your counter. When I have 900 exp in my counter but i'm in a team fight and then all my allies level up fighting, you don't get to use it. And the later levels are where the advantage from extra exp teeter off. End game the difference in exp from kill alone are greater than Zilean's passive old or new could ever hope to be worth.
Essentially the advantage of his passive is only useful early to early-mid game. Additionally having a bunch of exp accumulated you cannot use, does not help you. And when you finally use it and it only uses 200 up to get them from lvl 15-16, you only get 200 as well, leaving the other 700 sitting in your little counter and unable to attempt to use for 120 seconds, assuming when it ends you are out of combat, by an almost leveled up ally, and have the time to channel for 2 seconds (its and actual interrupt-able channel).
When you play Zilean, and actually learn how his new passive works, and see it in game. Then please come back and tell me how its not worse than it was before. This just in, I have over 100 Zilean games this season between my two accounts in ranked alone. Go talk about a champion you know