IHRA's big open question is "Is there any specific, controversial (left-wing) case, that some people contend is antisemitic, that IHRA would decisively conclude is not antisemitic?"
JDA's big open question is the opposite: "Is there any specific, controversial (left-wing) case, that some people deny is antisemitic, that JDA would decisively conclude is antisemitic?"
I say "specific" because both definitions say generally that there are things that are not/are antisemitic, respectively. For instance, IHRA says criticism of Israel similar to that leveled at other countries is not antisemitic, JDA says treating Jews-qua-Jews as agents of Israel is antisemitic. But these are generalities; the question is whether they'll ever cash out in a live controversy.
For IHRA, think about settlement boycotts or labeling requirements, or calls to condition aid to Israel. These are sometimes called antisemitic, but perhaps under the best reading of IHRA they should not be. But will people who promote the IHRA definition ever use it to exonerate? Will they ever say in a specific, live controversy, "no, contrary to what's being claimed, under IHRA that's not antisemitic, and the people claiming that it is are simply wrong?" And JDA raises the same question, albeit from the opposite angle. We wonder: will JDA proponents ever wield JDA to say, in a specific, live controversy, "yes, this is antisemitic, and the people denying that its antisemitic are simply wrong?" The cynics suspect the answers are no. In any remotely contestable case, nobody will ever be found innocent under IHRA, and nobody will ever be found guilty under JDA.
Are the cynics right? The JDA maybe has a good test case in front of it right now, with the controversy over David Miller, until recently a professor at the University of Bristol. Miller's defenders are, of course, contending that the campaign against him is the fruit of IHRA, demonstrative of IHRA's propensity to "ban all criticism" of Israel. But to my eyes, the Miller case is less a test case for IHRA than it is for JDA. Leave aside the academic freedom implications of him being sacked, which raise a separate problem (it is perfectly cogent to say that Miller is an antisemite and academic freedom nonetheless protects antisemitism). Is Miller -- who called interfaith programming between Jews and Muslims in London a Zionist trojan horse, who said all campus JSocs (Jewish Societies) are "pawns" and agents of the Israeli government who have been "directed by the State of Israel" to engage in campaigns of harassment and intimidation -- an antisemite?
For my part, I've often suspected that, among JDA drafters, David Miller was their imagined template example of someone "left" that is properly called antisemitic. That's pure speculation on my part. But it does make for a decent-ish test of a circumstance where JDA can and should, in a specific case, say "yes, this is antisemitic, and the people contending he was engaged in mere 'criticism of Israel' are wrong."
I hope JDAers pass the test. But make no mistake: a large portion of the JDA's constituency comprises those for whom its utility is solely in its capacity to deny antisemitism, not identify it. The temptation will be to hem and haw and hedge, or just stay silent -- a strategic ambiguity that lets JDA continue to serve as the avatar of antisemitism denial. If JDA folks do what they should do, and are clear and unambiguous that Miller is an antisemite, they will almost certainly lose a lot of their base of support.