"Anti-zionism" as a political phenomenon is puzzling to me. There are no comparative movements with respect to other countries - one does not hear critics of the Iraq war bring up 1776 to say that the American rebellion was unjustified, the revolution should not have happened, etc. Mostly everyone is fairly content to take the existence of America as a given in these debates and proceed from there; and yet with Israel there is the peculiar tendency to go back to 1948 and reargue the merits of the Israeli war for independence.
I should clarify that I'm talking here about anti-zionism in its practical sense, i.e. what people who have said they are anti-zionist have went on to say in their conversations with me. So I want to side step the issue of what anti-zionism is intellectually and whether its intellectual core actually implies anything like the first paragraph above. For practical purposes, I feel that my description is fair: all the people I have met who declared themselves to be against zionism went on, in the next few minutes, to argue against the propriety or wisdom of Israel's existence.
In fact, what I wrote in the first paragraph is not entirely accurate. There are movements against the existence of Transistria in Moldova; against the existence of Nagorno-Karabakh in Azerbaijan; against the existence of Abkhazia and South Ossetia in Georgia. There are a number of such examples associated with breakaway and semi-autonomous states all over the world. What all these movements have in common, though, is that they are more or less limited to the countries that want to capture back what they see as their territory, i.e. Moldova, Azerbaijan, Georgia in these instance. By contrast, anti-zionism seems to have a steady base of support in the United States and throughout the western world.
There is another difference - the above examples are all associated with recent conflicts whereas Israel has existed for close to 60 years now. As far as I'm aware, there is not a single other state which has existed for that long that still has a significant body of critics of its existence throughout the western world.