As far as I'm now aware just three known Jedi are still alive at the time of Rebels. Obi-Wan, Yoda and Kanan. The Jocasta Nu thing escaped my attention until now or I'd forgotten about her survival of Order 66, but either way it would sort of make sense that an elderly Jedi librarian who'd more or less sworn off using her lightsaber by the time of AOTC could disappear more effectively and blend into the background of the new galactic order more easily than a warrior prone to ignite their own weapon at the first sign of danger. Though very powerful in the Force Jocasta may have had a smaller and less proiminent Force "signature" than most other members of the Jedi Order due to leading an almost pacifistic life as the head archivist and librarian of the ancient Jedi records. She could well have slipped away from the Jedi Temple carrying important holocrons and other records with her as Anakin/Vader and his Clone Troopers were launching their massacre.
In the old EU, Order 66 was fairly effective. By the time of A New Hope less than one percent of the Jedi Order remains, and a part of that has turned to the dark side. By the time Luke starts up his New Jedi Order? Around half of the previous survivors are dead, and some of them are still on the side of the Dark Side.
In video games, Darth Vader usually kills everyone. Though, the same is true of Shaak Ti, who had several "deaths" in various iterations of ROTS video game, novel, and then survived in "Force Unleashed."
Though they captured and executed her eventually, Luminara presumably survived Order 66 somehow. IIRC Vos is also know to have survived as well, but nothing is known beyond that. Not sure if they really count, but the Grand Inquisitor and at least the initial recruits into the Inquisitorius were all former Jedi. It's unknown exactly when and how each of them were recruited by Sidious, but it seems to have been at some point prior to Order 66. It's not been fully depicted as of yet, but it's been established that Ahsoka was fighting alongside clones (including Rex, sans biochip) on Mandalore during Order 66, and had to fake her own death to escape. So it's not like she sat the whole thing out. The thing with Jocasta Nu is a relatively recent development in the Vader comic. I don't recall if it says exactly how she survived; probably either slipped out of the temple or was away on some academic mission. Either way, as an archivist she was never on the frontlines or among clones that often, so it's little wonder she escaped the initial slaughter.
In the comics there were actually Jedi who not only survived Order 66, they actually lived all the way up to the Legacy comic 100+ years after ANH. Spoiler: Jocasta Nu's survival I only read about it, but the most recent arc of the Vader comic dealt with him going after Jocasta and a Holocron that she took with her. The article said that the arc ended with a flash forward to Luke finding the Holocron, and that it is possibly one of the ways Luke learned about the original Jedi Order.
Short version: when he was still a Jedi he asked to be allowed to read the library's more advanced texts about the dark side. She told him "no". So clearly it's all Jocasta's fault.
Well I said one of the reasons, not the only reason :P But yeah, he didn't like how the Jedi kept secrets.
That was my thinking. Though, a shushing librarian with a lightsaber I would take a little more seriously.
It was less about the way the Jedi kept secrets in general and more about how he personally was denied access to something very dangerous that he wanted for himself (with good reason, it appears!) So it's more a symptom of his fall than a cause. Whatever other contributing factors there might have been, apparently what tipped him over the edge was the whole situation with Ahsoka's expulsion and Barriss's confession. Indeed, he was supposedly this guy right here and one of these four guys. So he had a front row seat to the whole thing.
Well, she could be a condescending bitch. I wonder if anyone ribbed her over her belief that Kamino can't exist if it isn't in the records?
I think they'd be more concerned at the security breach that allowed someone (most likely Dooku) to delete something from the archives. Keep in mind though that her perceived arrogance is less of a particular character flaw and more of a reflection of the arrogance of the Order as a whole. They've been the unchallenged keepers of peace and justice for a millennium, they have the largest most comprehensive library in the known galaxy. They've been everywhere, done everything. So it's not surprising that they think they know everything already. Also in fairness, 99.999999999% of the time any Jedi went to the head librarian saying they can't find something, it's not because a Sith deleted it, but either because it really doesn't exist, or they're not using the index search properly.