It was a big deal to Sokrates' followers, obviously, since at least two of them later took it upon themselves to write up publishable versions of his defence speech -- that is, the Apologies of Xenophon and Plato. We don't have any information on how big a deal it was outside that circle.

There is some reason to think, though, that the prosecution itself depended on a certain amount of public notoriety. Officially the trial was on a charge of corrupting the youth of Athens. According to Plato's version of the defence speech, Sokrates ironically generalised this to "introducing new gods", mocking his prosecutors by implying that they were really prosecuting the version of Sokrates depicted in Aristophanes' Clouds.

But there was a lot of public anger at several members of Sokrates' circle -- and for good reason. This made him a very obvious target as a fall guy for their appalling crimes in the previous few years. In particular, Sokrates had very close links to Alkibiades, Kritias, and Charmides. Alkibiades was a notorious traitor to the state; Alkibiades and Kritias were widely thought to be behind the mutilation of the Herms (sacred statues), an incident 13 years earlier that caused widespread panic; Charmides was found guilty of profaning the Eleusinian Mysteries a few years before that; Kritias and Charmides were at the centre of a coup d'état in 411 that overthrew the democratic government. Most importantly of all, after the end of the Peloponnesian War in 404, when the brutal regime of the Thirty Tyrants was appointed to rule Athens on behalf of the city's enemy, Sparta, Kritias was one of the tyrants and Charmides was an important minister. Kritias, in particular, was known for his brutality and violence.

Once that spell was over, Kritias, people like him, and people that associated with him, were vilified for very, very good reason. As far as the population of Athens was concerned they were brutal, bloodthirsty monsters. Sokrates was not only an associate, he was their teacher.

This may be one reason why the extant sources try to put so much stress on Sokrates' innocence in this time of atrocities, telling us that Sokrates disobeyed the orders of the Thirty Tyrants on one occasion. Still, it's not very surprising that there was widespread rage, and his links to the people responsible for this period of destruction, insanity, and death put him in a precarious position to say the least. At the time of his trial in 399 it would have been easy to see it as, symbolically, a trial for war crimes. And -- who knows? -- that impression may not be a false one, after all.