I would contend that, above all else, timing. Anything that breaks up an opponent.s rhythm or from setting up and in turn creates openings.
If it serves its purpose. Depending on gifts and style, a boxer is going to want to use his jab for different things. If he succeeds at accomplishing this goal a large majority of the time, I'd call the jab good.
Setting the pace of a fight and how versatile it is used. Can be used as a distance seperator or closer(great inside fighters could have solid jabs which gets overlooked because of the brutality of inside fighting). Can be used defensively to seperate space as well as offensively as a setup to bigger punches. All of this is not needed in order to have a great jab but are certain elements. Ali and Tunney had great jabs as outside fighters and Chavez and Duran had great jabs despite being distance closers. So yeah, to set the pace of the fight to do what you want to do and keep your opponent guessing as a whole is what I'd consider a great jab.(Was about to say fap whoops sorry!)
* Accuracy * Speed * Power/sting * Timing * Range * Versatility A jab doesn't need to have all of these traits to be a good or even a great jab. On the other hand a jab can have all of these traits in spades but it can still be a liability if the boxer overcommits to the jab. Ike Quartey had a tendancy to overcommit to the jab which could make his offense predictable and easy to time. E.g. his knockdown against Verno Philips.
Yes Buchanan's jab was great, ...I've always thought so, and so was the jab of the man in my avi,...Willie Pastrano could tattoo his name, address and zip code on a guy's face with that jab of his.