I'm Zionist because I, like Yglesias, live in a world where (mostly ethnic) nationalism remains the only apparent mechanism for 'self determination' and framework for peace available to most of the world. Nationalism, per se, sort of sickens me. But, genocide sickens me much, much more. And the multiple pathologies that accrue around anti-semitism--in America, Europe, Arabia, Asia--frequently drive me to angry despair, so I cannot talk about Israel with 95% of my non-Jewish liberal friends, with whom I agree on most everything else (I, er, get upset, and sound a lot like Marty Peretz, whose tone I frequently find abrasive, myself.)
I've been hoping against hope that J Street might turn out to be a strong defender of Zionism without confusing criticism of any particularly wrong-headed Israeli strategies (e.g., especially west bank settlements) with Anti-zionism. If it goes in the direction of Ben-Ami, I will join it. If it tacks toward the Walt/Mersheimer/The Nation left, I'm out.
As you can tell, I'm more passionate than bright about these matters, but I probably well represent the feelings of many Jewish American liberals.