It’s an old post, but I still stand by it.
Tl;dr, I think Zeratul should have less mobility but more sustain and durability. He should be committing to his engagements a bit more.
Beyond the 2 hard-counters I mentioned in that post, Brightwing and Uther, Zeratul has a couple of soft-counters like Malfurion, Morales, Diablo, Medivh, Zarya, and Yrel. I’d actually add Mei as to the list of the only 3 real hard-counters to Zeratul, just because of all the disruption and CC she brings to the table, but she wasn’t in the game when I made that thread.
Aside from them, there aren’t too many that can actually do much against a good Zeratul – most heroes don’t have fast enough CC, damage mitigation, or zoning potential necessary to ward off a Zeratul.