you dont need fancy programs to do Zettelkasten...
-kfitting
The question is what is "fancy" or "essential" in this context for a program to function as a Zettelkasten or function more effectively (making full use of the advantages computers offer to a chest of drawers with paper index cards).
One of the issues is whether a Zettelkasten becomes more effective the easier it makes the creation of direct links between notes (such as in a wiki) or whether it is just an added non-essential luxury. Christian argues that
You Don't Need a Wiki, while for Manfred enabling easier
Note Connections wiki-style makes a Zettelkasten fulfil its function as your external brain better.
I'd argue that some additional bells and whistles that directly focus on improving the capturing, retrieval, analysis, linking, and synthesis of notes can make a big difference to a Zettelkasten, especially as one of its purposes is to discover new relationships between notes, which is easier if you have some alternative ways to organise and visualise the data, rather than just have a flat list of plain text notes that aren't even linked directly.
The other debate here is whether OneNote could be a good Zettelkasten (as IainB argues above), or its
skeumorphism gets in the way, as Manfred argues, and who prefers ConnectedText, a desktop wiki instead.
As for OneNote, I still haven't seen an example or explanation of how it handles thousands or tens of thousands of notes, and in the absence of that I have a hard time seeing how it would work as a Zettelkasten in the long run.