Well, this was a short one. :)
From the very start of its existence, ZFS supported case-insensitive datasets. In theory, if you share disk with Windows machine, this is what you should use. But reality is a bit more complicated. It's not that setting doesn't work. It's more a case of working too well.
Realistically you are going to be running ZFS on some *nix machine and access it from Windows, it'll be via Samba. As *nix API generally expects case-sensitivity, Samba will dynamically convert what it shares from the case-sensitive world into the case-insensitive one. If file system is case-insensitive, Samba will get confused and you will suddenly have issues renaming files that differ only in case.
For example, you won't be able to rename
Test.txt. Before doing rename Samba will if the new file already exists (step needed if underlying system is case-insensitive) in order to avoid overwriting unrelated file. This second check will fail on case-insensitive dataset as ZFS will report
Test.txt exists. Because of this check (that would be necessary on case-sensitive file system) Samba will incorrectly think that destination already exists and not allow the rename. Yep, any rename differing only in case will fail.
Now, this could be fixable. If Samba would recognize the file system is case-insensitive, it could skip that check. But what if you have case-sensitive file system mounted within case-insensitive dataset? Or vice-versa? Should Samba check on every access or cache results? For something that doesn't happen on *nix often, this would be either flaky implementation or a big performance hit.
Therefore, Samba assumes that file system is case-sensitive. In 99% of cases, this is true. Unless you want to chase ghosts, just give it what it wants.